Without exhaustive knowledge of canon, there's no way to know whether a given detail is specified at all or not.

That's a good point. We can only hope that mistakes will be obscure.

Eliezer may not have realized that Harry and Draco's experimental results should have been outright confusing given the canon-available information on inheritance.

But that's just DidNotDoTheResearch. One obvious source of information begins (and has begun for some time) with the most relevant quotation.

Scabbers!Pettigrew requires one person to have done something ridiculously stupid, all on his own.

Sadly, that is very credible in the world that I know. (The first mistake is driven by panic; then he sticks with it out of intellectual inertia, sunk cost fallacy, etc.) However, it violates the rule that Harry's enemies must also be more rational than in canon. But then Eliezer still keeps some characters (Hermione, for example) canon!stupid.

The point is not that Eliezer chose wrong but that it's unpredictable.