Every time I've ever read or discussed this, the answer was that I had
to establish the communications with the Class C ATC. If it was not
established, do not enter.

With an instructor, I was leaving SSF (Stinson, Class D ) and going
back to SAT (San Antonio Class C) , and as was your situation we could
not get 2-way established due to traffic. The instructor has us do a
180 so as not to intrude on the Class C improperly.

That pretty much established the rules to me; I believe you could be
busted.