Perhaps I wasn't direct enough: putting it on the capture list only works for local captures. This same notion is also important on properties, functions, subscripts, and anything else that's potentially used from outside the actor's isolation domain.
Most of the rest of your note is essentially reasons why you want async let to be split out from Structured Concurrency, with many claims of it having special cases (most of which are wrong). The second pitch ensures that async let is presented as syntactic sugar over task groups so we can reason about just the one thing, understand what the syntactic sugar buys, and remove the syntactic sugar if that's the right thing to do. I don't consider going point-by-point to be useful to move the needle forward here.
Doug