When to acknowledge ATC

When they first created the alphabetic areas, I was taught that Class
B airspace required a specific clearance prior to entrance..... thus,
something like "N12345 cleared to enter Class B airspace" as opposed
to just "establishing" two-way communications as required in Class
C.... and just barging in.

You are right, a clearance is required (as opposed to just establishing
communciations), however, the words you quote are NOT the only way that
clearance is conveyed. It's up to the pilot to be sure they have a
clearance.


As for readbacks... As for VFR, I'd have to admit that I'm really not
clear on the absolute requirements, though I tend to read back
everything that shouldn't be obvious or ambiguous....

You NEVER have to readback anything back IFR or VFR with the sole
exception of runway crossing/holdshort instructions. Even then,
the job of insisting on the readback is the controllers, not the pilots.