Oh boy..well I'll try to answer best I can.

1&2) CC is used yes, but from what I've experienced (and Im a level 50 whm) it's not a matter of life or death if you don't unless you have a group that pulls an extra group. As far as who uses it I only know of thm/blm and whm using sleep, I haven't really played other classes yet so I dont know of other cc's sorry

3) Shards are pretty easy to come across. If you dont need them then selling them was a good decision.

4) I would only do one job at a time really, for story/side quests, once you get 1 50 you get an armory bonus to help you level all the other jobs since you used up all the one times quests/story to get your first 50

5) As far as I know it shouldn't matter whether you are a basic sub or not, as long as you continuously sub you should get vet rewards. I may be wrong.