Too tired to go point-by-point in quotes, so brief reply:
- Drain was a bad button. Removing it shouldn't have had any real impact for the vast majority of players. My experience may not be the same as that of the next player, but if I managed to forget it existed for so long, I imagine I wasn't the only one. Not once since Drain was introduced have I ever thought to myself, "Man, I wish I'd had Drain on my hotbar for that thing I was doing." Its removal doesn't represent any significant dumbing down of gameplay, though I do recognize that at least some players would have liked to retain it as an option.
- Big difference between inexperienced player confusion and the issue of players who should know enough to know better but still refuse to read their tooltips and make reasonably sound gameplay decisions. The former is more descriptive of the RDMs who think that their job is to heal whenever party members lose HP even when healers are present and active (because, to their way of thinking, why else do they have Vercure if not to use it?), while the latter is more what you already described.
- The chocobo comment was offhand, because no one actually has to use one, but it's also nothing that requires excessive grinding or extensive game knowledge to acquire. I don't think it's actually possible to miss obtaining your chocobo companion and still advance in the original MSQ, is it? In any case, it was a reference to another, more efficient way that players can get reliable healing for solo content if desired without wasting time with stuff like Drain.