I'm going to try my hand at Q1, but I'm a 0L who knows nothing so someone correct me if I'm wrong.
I wouldn't say that reliance/promissory estoppel is a literal substitute for consideration. Instead, it's a way of protecting an offeree's reliance interest in situations where consideration can't be found. So precisely because you can't get damages for the expectation interest, you want to try and find consideration before moving to promissory estoppel.
Also, Drennan expanded §90 to create what's now §87 -- it's a case where reliance on an offer for a bilateral contract made that offer irrevocable (basically, in this situation an option is created). If the subcontractor hadn't tried to revoke before acceptance, there would have been a perfectly good consideration-based contract, as opposed to a scenario where the promise wasn't supported by consideration at all.