You're actually taking the word of <PERSON> (who lies and charms his way to get things done) very seriously.
For all we know, <PERSON> could be lying. Here are a few reasons why I think <PERSON> is lying
1. GG did not know about <PERSON> - In 'Fantastic Beasts', GG only thought <PERSON> was a squib. He only realised <PERSON> was an obscurial and his power later in the movie. And over a short period of time(events of part 2) <PERSON> finding out roots of <PERSON> seems again far-fetched.
2. Too much coincidence of <PERSON> 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 <PERSON>.
3. GG needs <PERSON> to kill <PERSON> - Most importantly in part 2, we see that the reason <PERSON> needs <PERSON> with him is to kill <PERSON>. So to get <PERSON> to do the job, <PERSON> could be building up a story to get <PERSON> to be personally hating <PERSON>. <PERSON> is charming and convincing like that
4. <PERSON> knows everything- It would be impossible for him to not know of a person related to him. 725
So if <PERSON> knew of <PERSON>, he would have tried to get to <PERSON> first to protect him
5. Timelines don't really make sense - <PERSON> was 47 when these events take place and <PERSON> was around 20 years old. So there is at-least a vast difference of 27 years between both of them. So <PERSON> would need to be born when <PERSON> 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 <PERSON> could be long lost (unless <PERSON>'s age is lied about - which is far-fetched again)
6. <PERSON> 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 <PERSON> would mess with this. <PERSON> would have dug up this like ages ago for her book.
So the final conclusion NO, <PERSON> did not mess up and that <PERSON> most probably was lying to get <PERSON> to kill <PERSON>. And that <PERSON> most likely is not a <PERSON>, we should wait for the next part before berating <PERSON> :)
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 <PERSON> or <PERSON>) 725