Ok, first off you can edit your posts to get around the 1000 character limit. It's dumb, I know.

1: Yes, they are basically guilds. Finding a nice FC can make the game much more enjoyable, but it's not really required per se.

2: Yes. You should be doing all your class quests for any class you level up anyway, and BLM is the evolved form of THM.

3: Yes. You get a new job quest every 5 levels until 50, at which point they come every 2 levels.

4: Try to keep your gear from falling too far behind. My general rule of thumb is that if your gear is 10 levels out of date (or 4 levels past level 50) you should focus on getting new ones. The sole exception is that some level 50 sets (notably the augmented ironworks gear) have a higher item level than the post-50 crafted gear, and as such can last you a considerable time into the expansion.

5: Palace of the Dead is the fastest way to level DPS classes as it doesn't need role requirements, and an option for all jobs to avoid caring about gear. I would recommend pausing every now and then to practice using your new abilities as you gain them however, so you're not thrown into level 60 with no idea of how your class works. Also, tanks and healers get fast queues for leveling dungeons, so it's far faster for them to level that way.

6: No idea, I level in dungeons because I prefer them.

7: AST is currently preferred, but WHM is far from useless. Also, Stormblood is launching in a little while and all classes are undergoing some major changes in how they work, so "what's good now" is irrelevant.