I'm letting a friend of mine borrow Resident Evil 2 from me and I was explaining to him how the scenarios worked. I told him in he should play Claire's game first to unlock Leon's B scenario. Correct me if I'm wrong but isn't that how RE2 plays out canonically? Since Ada is supposed be "killed" by Mr. X.

Correct, although some elements of the other scenarios are also taken to the canon.

Claire confronting Birkin. Seen in a flashback in Degeneration. It's kind of like in RE1 the canon story is a mix of Chris and Jill's scenarios.

Yeah but in Degeneration they showed both Claire and Leon working together fighting off Birkin when in fact it was just Claire. Same thing goes with Umbrella Chronicles. Chris and Jill didn't travel together throughout the mansion. If anything it was Jill and Barry and Chris and Rebecca.

The real canon is pretty vague for sure. Most of the events can be mixed to make up the real canon.

The RE storyline has so many plotholes and inconsistencies, but is still awesome.

It's very hard to explain. But do you really think that only Claire confronted Birkin? Honestly?

No it didn't. Nothing has ever said that Claire A and Leon B were canon. It was just one possible timeline within that game. It is very unlikely that in the official canon, Claire confront Birkin alone. No one can be proven wrong or correct at this point in time though.

Claire A, Leon B is the only way RE2 can be presented canonically because that's the only way Ada could've survived to go on to appear in RE4.

That one small part of that scenario is canon, but that doesn't mean the whole scenario is canon. It's just like RE1.

In a REtrospective from gametrailers, they say that capcom has officially confirmed that Claire A Leon B is canon. I'll give a link to the vid if you want.

Please do. If it is officially canon, I will not be pleased.