1. I dont think skipping side quests is good or bad advice but I personally would do them until you dont need anymore. Saying a few measly sidequests will help you lvl a second class faster is not very accurate considering youd have to go back to those regions and they barely give 5% of a lvl. You can do guildhest for xp bonus on each class you lvl, along with hunt logs, dungeons, fates, and guildleves for the challenge log, lvling a second class is not as hard as some want to act like.
2. No, there is no mid level roulette. But as said before you get a pretty large chunk of xp by just doing the daily one time for the bonus, then selecting the dungeon you want to queue for so you dont get stuck in sashtasha all day.
3. Yes, you got something you probably did not notice. Equipping a job stone at lvl 30 will boost your primary stats for that job, think of it like a free mind ring for whm lol.
4. Player preference. No it will not ruin your game by not lvling them, you are not missing out by waiting til later. But lvling crafts lets you repair your own gear in the field and dungeons, and when repairing your own gear it goes above 100% durability.
5. Im good, thanks for asking.
6. Player preference again. It doesnt matter if you rush to the end or lvl them all to 40 before moving on. Do whatever you are having fun with.