I've been turning this TL over in my head, so I haven't formulated answers to your questions yet. Rock is broadly split up as IOTL, though as Micky Dolenz says, it takes a little longer for the pop/rock divide open up and serious rock critics are a little less inclined to sneer at pure pop, just in case someone surprises everyone like The Beach Boys did in 1967.
There should be a guest post coming about what's happened to the Bee Gees and I'm pondering a different career for Roy Wood in the late 70s. Also, there's another life I've saved that I haven't mentioned yet.
No thoughts yet on musical theatre or soundtracks, though it's worth mentioning that I'm trying to keep in mind that TTL and my
Doctor Who TL are both the same universe and eventually in that TL, I'll reveal that the Bond films have diverged in the 80s.