WotC have a very flexible approach to canon, which I find can be frustrating sometimes and a relief at others. There's an article on the subject on the FR wiki at https://forgottenrealms.fandom.com/wiki/Forgotten_Realms_Wiki:Canon that I found quite helpful.
Oh, and yes my understanding is that "Tav" indeed was inspired by "Gustav", though that's not to say that Larian didn't consider that the name might be fitting for other reasons. When I first saw it (before I heard about Project Gustav) I wondered if it were connected to the Wheel of Time
Ta'veren which would be quite fitting in some ways if entirely the wrong universe!
