If I may add my two cents...
It's crazy to think that Dennis isn't playing those drums on the Surfin' Safari LP. Someone once said "The least discerning musical ear will realise that Dennis' drumming was basic." Heads You Win-Tails I Lose...that could be nobody other than Dennis. Surely no session guy there.
Yeah, I don't think anyone's ever doubted that Dennis did almost all the drumming on the first two albums. But the myth of Brian using session drummers exclusively from 1963 on first appeared in David Leaf's book. I think someone must've told Leaf that Brian started using studio musicians on BBs records that year (which is entirely true - but not exclusively true). Leaf took it as an absolute, when in reality, the proof is that it was that the studio cats mostly AUGMENTED the BBs at first, but aside from a couple of times (such as "Why Do Fools Fall In Love"), they didn't actually REPLACE them, until the
Today! sessions....and even then, it wasn't a total, exclusive replacement.