1.

The key answer on the diner, dealing with a major theme of the movie, comes in the form of the quote of the old farmer (or rancher) who the police question at the diner:

"Those men robbed the bank that's been robbing me for 30 years."
^{-Paraphrased from the movie}

These people hold some admiration for the bandits, and certainly aren't going to sell them out to the bank that's been bleeding them dry for decades. They aren't bound by the letter of the law, but by their personal codes of ethics.

It's helpful to know the history of

"bandits rob with a gun, bankers rob with a pen"

featured in Woody Guthrie's "Pretty Boy Floyd", a song about a depression-era bandit hero. During the Great Depression, people couldn't pay their mortgages and were oppressed by the banks. Outlaws such as John Dillinger and Bonnie and Clyde were folk heroes. Hell or High Water is set in the wake of the Great Recession caused by a real-estate collapse based in large part on predatory loans. This directly relates to Toby's need of cash to pay off the predatory reverse-mortgage the bank tricked his mother into signing so they could effectively steal her land.

The waitress's refusal isn't inexplicable either. She's attracted to Toby, and even though he gently rebuffs her, he still tipped her $$$. By contrast, the cops are the guys taking that $$$ that she needs to take care of her kids. This violates her sense of justice to the degree where she berates the cops. Her determination of who are the good guys and the bad guys is not based on the laws of the state, but on her own moral compass.

2.

On number 2; maybe, but it would be hard to convict based just on thinking it was Toby's voice. There was nothing about them that particularly stood out, so the potential base of possible suspects would be very large.

3.

On number 3; they specifically only rob money from the trays because it's unmarked^[1]. The banker receiving the money could be suspicious, but he can't prove anything.

Also, Toby was advised by his lawyer to open up the trust account at that bank to keep things as copacetic as possible. i.e. the greed of the banker/bank works in Toby's favor - they'd rather have the cash back in an account with their bank than get into a lawsuit with an unknown outcome. The bird in the hand over the possible two in the bush, so to speak.
Toby's lawyer is also operating under his personal code of ethics, taking on risk beyond his financial gains in service of what he considers justice, against the letter of the law.

4.

On number 4; the assumption is that there is a reasonable amount of traffic. Texas is a massive state and you're talking about fanning out in all directions. Even at a 50 mile radius, that's a huge amount of territory to cover. Toby didn't look suspicious enough to be detained or searched further.

5.

On number 5; I don't have an answer, but as careful as Toby was trying to be, what's to have stopped him from buying the car through a 3rd party or from some source that's not easily traceable? They were old cars after all and it's a rural area. Maybe it was unregistered.

Code of Ethics

But the film isn't concerned with the legal aspects. It isn't at heart a police procedural, but a Western, and Westerns are concerned with personal codes of honor. It's really about the conflict between Toby and Officer Hamilton's individual codes.

Hamilton is presented as a competent cop. If the evidence isn't there, we have to assume Toby covered his tracks well enough.

Tanner's sacrifice of himself also probably contributes to Toby's free status at the end. His death gives the Rangers a partial win, and Hamilton can't finger his accomplice.

Tanner's self-sacrifice also arises out of his own personal code. He doesn't explicitly state his reasons for choosing to die in a standoff, which is partly related to his self-identification as a modern day Comanche, defined in the film as "Enemy to Everyone", but it is presumed his motivation is also to protect his brother and nephews, gaining some degree of redemption for his wayward life.

As to Toby not getting caught; he is actually caught — it's just that it's outside the scope of the law.

In keeping with the theme, Hamilton doesn't care if he can convict Toby in a court of law. Hamilton's code calls for vengeance regardless.

Hamilton becoming himself an outlaw, just as Toby had been compelled to do, validates Toby's choice and is the final reinforcement of the idea that justice and the law are not always the same thing.

Where it becomes really complex is where sympathies lie at the end of the film. Although Hamilton is grieving for his slain partner, there remains the question of whether Toby can be held responsible for his brother's actions. From the standpoint of the viewer, Hamilton may be seen as the villain, exacting retribution on a good man for a killing he did not commit, robbing Toby's sons of their father.

Toby's stated willingness to accept the consequences of his action is a final validation of his innate morality. This idea is at least as old as the death of Socrates, and is commented on more recently by MLK in his Letter from a Birmingham Jail.

Fifth, their final car (the one that exploded) was bought from a dealer. This car's characteristics were known, so why not look for a used car dealer who had sold such a car recently?

In the scene at the car lot, it was pretty clear that they were using a bribe to get the final car. The dealer said something like, "I won't report it stolen until Monday." meaning that he could pocket the bribe and never report that he'd seen the brothers.

To me, the biggest question comes from the last robbery where they had all those guys who were at the bank shooting at them as they fled and then chasing them until Tanner starts shooting at them. There very clearly were two bank robbers that everyone there saw, so it's very hard to believe that the police would just drop the second robber from any consideration and fail to very seriously investigate Toby and put together all these clues that he was the second robber. Also the FBI might not get involved in petty thefts from banks that only operate in one state, but when a double murder is committed in the course of a robbery, at that point it would seem they would get involved.

I think they explain why Toby wasn’t caught pretty well. In the restaurant the only other eyewitness, minus the waitress (who obviously wasn’t going to tell even if Marcus didn’t take her money, she was obviously interested and Toby and wasn’t snitching) was the old man who said he didn’t recognize Toby's face.

Think about it like this: two men sit in the same packed diner as you, bring zero attention to themselves, eat and leave. Later, these men are accused of robbing a bank. If the police come to you and ask you if you remember their faces in detail - enough detail to potentially put them in jail for a long time - are you going to be able to say with exact certainty that you remember without a shadow of a doubt? Most likely no.

Honestly, this has probably already happened to you - I’m sure you’ve been in a restaurant with someone who’s committed a crime and you never had any idea, probably even saw their face on the news and never even thought twice.

Also remember that Toby was always the one doing crowd control in the one robbery and he was always kind of standing back away from the teller while the teller had a gun in their face, which is a highly stressful situation. Right now, people are walking around with masks that cover half of their face every single day. If a criminal had one of those masks on, you could walk right past them on the street, and if you saw their full face plastered on the news, you’d never recognize them.

Thirdly, Marcus’ boss says the bank has been less cooperative than Toby's attorney, and it’s like Toby's attorney said: Toby should use the bank they robbed to set up the trust because it will remove suspicion. The bank got all their money back in the same week that they were robbed, and made money, and will continue to make money managing the trust. You see that these banks are in small poor towns; Toby is likely one of the wealthiest clients or will be making the bank more money then probably the entire town's residence combined. They say in the movie that Toby set up the trust with that specific branch and asked all kinds of questions and looked into it and the bank would not cooperate.

I think money is more powerful in this movie than you realize. If I came up to you tomorrow and said "Hey, I’m gonna rob you for $10,000 today, but in return next week I’ll give you your $10,000 back and $10,000 a month for the next 20 years", are you going to go to the cops and tell on me? If you’re smart, you won't. The power of greed is overwhelming. Honestly, I think the bank is suspicious that he stole their money, but they just don’t care.

Fourth, I don’t know if you’ve ever been to a roadblock, but they cannot search your vehicle without suspicion. There was still a gunfight going on. They didn’t even identify that the shooter was his brother, yet he hid his wound, handed over his license, and had a good backstory about what he was doing and where he was going. They had no reason, or better yet, no probable cause to search his vehicle, and even if they had, without probable cause the whole case would’ve gotten thrown away. Due process is still a thing.

The point of a roadblock is that they usually have a better description of a suspect. All they know about Toby is maybe his height and his accent, which is the most common accent in Texas.

Also Toby went to a different dealer for every vehicle and paid cash every time, so the cops had no idea that the vehicle Toby was driving was used in the robbery; they only knew about the Ford Bronco that his brother was driving. So really, all they have is that Toby was maybe in the remote area of the robbery at that time, nothing else.

Also, remember the Ford Bronco is the only vehicle to be identified out of all of the vehicles they used, and now that it’s been blown up, even if they traced it back to a dealer they had nothing to reference it to. They could ask the dealer if they sold Toby a Ford Bronco, the dealer would say yes, Toby could say it was a different Ford Bronco, and there’s no way to prove it because that Ford Bronco is torched. There’s no way to pull the VIN or anything as it was melted. I mean, sure, there’s witnesses to determine color and whatnot, but there is no physical hard evidence.

Fifth: all of the evidence that you’re bringing up here is all circumstantial. There’s no prints, there’s no hard evidence linking Toby to the crime. The problem with circumstantial evidence is that you poke one whole in it and the whole case falls apart. Which is exactly why Marcus’ boss tells him there’s nothing they can do and she’s even convinced Toby is clean and didn’t do it. You have to understand that they had no reason in the world to suspect Toby, meaning there was no reason to launch an investigation into him. I will say that if they'd found one piece of evidence that did link Toby, I think he would’ve gotten caught.