Is there any explanation for the differences to the existing Harry Potter timeline?

You're actually taking the word of Grindelwald (who lies and charms his way to get things done) very seriously.

For all we know, Grindelwald could be lying. Here are a few reasons why I think GG is lying

1. GG did not know about Credence - In 'Fantastic Beasts', GG only thought Credence was a squib. He only realised Credence was an obscurial and his power later in the movie. And over a short period of time(events of part 2) GG finding out roots of Credence seems again far-fetched.

2. Too much coincidence of Credence being a Dumbledore- It is too much of a coincidence that a random person GG enlists to do his work turns out to be a Dumbledore.

3. GG needs Credence to kill Dumbledore - Most importantly in part 2, we see that the reason GG needs Credence with him is to kill Dumbledore. So to get Credence to do the job, GG could be building up a story to get Credence to be personally hating Dumbledore. GG is charming and convincing like that

4. Dumbledore knows everything- It would be impossible for him to not know of a person related to him. So if Dumbledore knew of Credence, he would have tried to get to Credence first to protect him

5. Timelines don't really make sense - Dumbledore was 47 when these events take place and Credence was around 20 years old. So there is at-least a vast difference of 27 years between both of them. So Credence would need to be born when Albus was around 27. But during this time his father was in Azkaban and his mother and sister were already dead. Which totally rules out the fact that Credence could be long lost (unless Credence's age is lied about - which is far-fetched again)

6. JKR would not mess with such a big character's solid story The house history is fixed, the characters and their backgrounds are fixed. I don's see why JRK would mess with this. Rita would have dug up this like ages ago for her book.

So the final conclusion NO, JRK did not mess up and that Grindelwald most probably was lying to get Credence to kill Dumbledore. And that Credence most likely is not a Dumbledore, we should wait for the next part before berating JKR :)

And 2, the blood pact is not a mess up, it could be a part of their story we are discovering in detail (we earlier just knew very little about Dumbledore or Grindelwald)

Possibly, But Not Necessarily...

There are a few theories out there that could explain the BIG plot twist at the end of the movie that may pertain to THESE minor plot twists, because of whom the source of information is (Grindelwald), may not validate the information given.

Theories about BIG plot twist:

1. Percival conceived a child with another women while in Azkaban.

2. The character in question could be a cousin rather than a brother.

3. Grindelwald used dark magic to to play a trick. (Thus Grindelwald is an unreliable narrator.)

In Support of #3 (Different Perspective Than Previous Answer)

There are some callbacks in this film to The Goblet of Fire, which is also about dark magic creating a once thought-to-be impossible situation. (AKA: Harry's name is put into the goblet through dark magic, so the Phoenix visiting "So and So" could also be some kind of dark magic trick/illusion. It's also a bigger introduction to love triangles that begin to threaten "the team")

Note: Cho Chang is Harry's love interest in The Goblet of Fire/Order of the Phoenix (film version). Although the character is suppose to be either of British Asian or Irish Asian heritage, actress Katie Leung is Scottish born and of Hong Konger decent and her Scottish accent is clearly heard in the films. I brought this up because it bridges together thematic aesthetic and more through lines between The Crimes of Grindelwald & Goblet of Fire with love triangles & tragic death, imagery in lake with Scottish Kelpies (Loch Ness Monster), and Asian culture/folkore with the Chinese Zouwu.

There is also this "French Connection"...

However, I feel that most references tend to callback Order of the Phoenix, especially with such a similar narrative of "a call to arms", but that film relies a great deal IMO on the events featured at the end of The Goblet of Fire, such as the return of Voldermort and the death of Cedric Diggory, even tying Harry being able to see Thestral due to that event.

I feel that, although more adventurous, Fantastic Beasts and Where to Find Them is in similitude with the first two Harry Potter films being more kid-friendly and lighter on the darker elements. The Crimes of Grindelwald is more like a mash-up between Prisoner of Azkaban, Goblet of Fire, and Order of the Phoenix.

I think this all points to an idea that viewers shouldn't trust Grindelwald and the information he passes onto the audience.

About The Blood Pact

J.K. Rowling has this habit of introducing objects or other things of interest that doesn't get fleshed out until much later. In Example the Triwizard Trophy is introduced as the first unexplained portkey at the end of The Goblet of Fire. It may then be possible there is some catch 22 with a blood pact that we don't understand.

Some *speculation I have come across is that, it only makes sense that the blood pact happened before the duel, but perhaps Albus was only blocking spells between Aberforth and Grindelwald whom may have already been actively engaged, as opposed to Albus being the one offensively dueling another. There could be a catch 22 about dueling.

A simpler explanation that others believe that maybe he just tried dueling anyways and the spell was rebounded due to the blood pact and accidentally killed Ariana.

At any rate there could surely be reasons, as opposed to Rowling directly retconning