Addendums to the words of others...

1. I'd say no. As has been mentioned, EXP gains have been massively enhanced across the board for non-FATE activities, so there's no real shortage of EXP anymore like there was in 2.0. Especially now that daily roulettes exist, which can give you significant EXP boosts once a day.

2. Go to 30, and then level whichever class is required to 15 for your Job, and then finish taking it to 50. That's my suggestion, but realistically, the only 'mandatory' process is to be sure you get your Job - you really, really don't want to skip it. That said there's absolutely 0 necessity to do so, I just find that maxing one job lets you focus the EXP Bonuses on other classes you want to work and still having progression stuff to do on that class simultaneously.

3. Yes and no. CUL always sell food, and now the other classes at 50 have the ability to make Glamour Prisms that all characters can use to alter their gear's appearance, so there will always be a demand for crafters now. There's also some fairly high level craftable gear if you get really good that people still occasionally buy from, although usually people just farm tomes to gear up. If you're not a level 50 crafter however, you'll find that your creations aren't nearly as valuable.