1. I'd recommend gettin' all the expansions and add-ons. Try gettin' the Ultimate Collection from Steam for cheap.
2. Yes. You'll have to have access to all 9 Abyssea areas to get Shinryu access, and if you want all your EAF pieces.
3. You can enter Abyssea at lv30, but you won't be doin' much except opening treasure caskets until at least lv75.
4. Depends on what you're after. I don't have PUP leveled, but I'd imagine you'd still want a WHM with you. PUP and WHM could probably duo a lot of stuff.
5. Even if you're not goin' to jump right in to Abyssea content, do all the quests early on so you can get your traverser stone count buildin'.
a. One of your early goals should be gettin' all of the celerity, merit, furtherance, sojourn and avarice abyssites. Probably in that order.
b. If you don't have an LS, you'll probably have to rely on /shout parties for zone bosses and other things. Hang out in Port Jeuno and you'll see plenty.