1. Yes, it's permanent - abilink to a friend and you have access to their job abilities, so you could level up different jobs and access each other's progress, essentially halving the time to level everything up. It's one character per friend, but the game will give you bot friends if you don't have enough real life friends playing.
2. I think when you choose Send you have the ability to change what you're sending, but I never used it so I'm not 100%. Edit: yeah if you select Send you then pick a command from your usual battle menu.
3. Not sure about earlier than this (I did some grinding by mostly just running about wherever I was at the time) but there's a smallish island in the northeast of the map that has a beach on the south end and ice up north. The beach is excellent for grinding, and accessible after
.
4. You do, a bit. Couldn't say exactly how long you'd have to stand in the shop though!