at minimum to have read all the books

I find it odd that Eliezer writes this fic without having read all of the books, but it's his life and his fic. And for the purposes of complying with canon, I believe that careful perusal of Wikipedia and other fan-made sites should be sufficient.

The biggest danger is poor characterisation of characters who don't show up in what Eliezer has read. Reference sites are good at explaining what people did, but poor at describing what they were like. However, I don't think that Eliezer is using any such characters, is he?

And therefore, I agree with you that it would be nice if Eliezer spelt out his points of departure; having not read all of the books should not stop him. Unfortunately, I think that some of his rules (such as the rule against anything to stupid for a book for adults) are pretty open-ended. (And how to apply them is very unclear; why is Peter Pettigrew as Scabbers stupider than the rules of Quidditch, for example?)