What was the point of the fake heist?

This is a lot of questions, but I'll try to answer them one by one.

1. As there were no card transactions available on the day of the heist, the vendors at the stadium were too busy to count the money before sending it through the tubes, therefore there was no way to know exactly how much money was in the vault when it was robbed. This allowed the Logans to still keep a significant portion of the money (stolen by throwing the trash bags into the dumpsters and recovering it later from the dump) whilst making everyone believe that it had been recovered entirely, including the authorities and the Bang brothers.

2. The Bang brothers were insistent on taking a significantly larger cut of the profits than the Logans wanted them to, so by returning some of the money and keeping the rest, they likely came out of the heist with more money than splitting the entire amount between themselves and all of the brothers.

   However, they made sure that everyone who was directly or indirectly involved with the heist (including the inmate who incited the riot and the teller who received the cake) were paid. They also paid Joe Bang back by replacing his "nest egg" that was buried under the tree at his house and later stolen from him by his sister-in-law whilst he was in prison (He didn't know that Jimmy was the one who had phoned in the tip about where the truck with the money was).

3. Jimmy went underground because he knew that he was the one most closely connected to the heist, having worked at the stadium before and having the least concrete alibi. He was also hiding from the Bang brothers, as they thought he had lost all of the money they had stolen. He mentioned at the end that he knew he was free and clear when his phone went off, as he knew the FBI were tapping it because he wasn't paying his bill yet it was still working. I'm guessing that Joe Bang also didn't care that Jimmy had lost the money by that point, as he had his nest egg money anyway.

4. It was insinuated at the time that the Speedway was being shifty by committing insurance fraud, as they recovered the stolen money on insurance even though everyone believed it had all been returned. However as we learn later, only a portion of the money was returned, therefore it was likely that the calculations of the "finance guys" working out how much money the Speedway had made on race day were actually accurate, as they calculated that there should have been more money than there was in the vault, but at that point the Speedway didn't care because they had recovered the shortfall from the insurance.

5. The FBI agent was the only one who had all of the information needed to piece together what might have happened, as she had an eyewitness and knew that the prison was hiding something, even though all of the evidence she had would not have held up in court (all the suspects had reasonable alibis for the heist, the money was supposedly returned, and their only witness was directly contradicted in his account of what happened). However, she would not have wanted to let the investigation go even though it was officially closed, so thought she could try to get some evidence watching them directly.

   It also ties into the whole point of the "Logan curse", as Clyde believes that everyone in the Logan family has something bad happen to them just before they are about to succeed (he got his hand blown off on his way out of Iraq, Jimmy injured his leg just before he made it big in football, their grandmother accidentally washed a winning lottery ticket).

   The audience were supposed to believe that Max seeing Clyde just before they got out of the stadium after the heist was this curse in action, which was overturned after Dayton White rendered his statement useless by saying it never happened. However, with the FBI agent going to the bar just as they were celebrating about getting away with the money, it suggests before the movie finishes that this might be the curse working, as she could potentially catch them just as they think they have gotten away with the heist.

I've just seen the extra question in the comments as well about the smoking bearded guy:

6. His name was Earl. He was also the garbage truck driver. And he is seen smoking outside the bar at the beginning. He gives Clyde a light so that Clyde can firebomb Max's truck, so he is obviously a confidant of the Logans. He was there to provide a reason for there to be smoke coming up through the tubes after the explosion. I'm guessing that he wasn't shown as an original part of the plan in order to provide extra suspense for the audience, as the smoke provided a reason for the guards to be investigating, and the guy was only discovered to be smoking once the heist was essentially completed.