A) Look at this list for the preferred servers:
Regarding Character Creation Restrictions and Statuses of Congested/Preferred Worlds | FINAL FANTASY XIV, The Lodestone . Assuming the promotion for bonuses is still going, then any of the preferred server would work. You can tell by logging in, if you have a Road to 60 buff(double xp), then it'd be still active, not sure it is still, I don't remember seeing it being turned off but I haven't played in a while.
B) As long as you don't start in Limsa Lominsa(MRD/ACN/THF) then you can run to the other place, however your quests would still be in the original place until they merge. I'd recommend making a Lancer or whatever you want to start in Gridannia also, and then leveling gladiator quickly once you reach the first dungeon place. Takes only like 10mins really to get to 15 on a second class(you get 100%xp bonus for any class that's not your highest, until 61 then it's only 50%) so you can do it quick before doing the first dungeon, probably as long as it takes to run your ass from one city to another without a mount.
C) There's no advantage to other classes now, you don't need cross class skills(you just get a pool based on your class' role, so all tanks get the tank skills regardless once they reach the appropriate level) and you also don't need a secondary class to 15 to unlock jobs anymore(WHM just needs 30CNJ instead of 30CNJ+15THM). As for the best tank, assuming your girlfriend isn't the best healer around which if she's not too used to video games is mostly going to be the case, PLD is the best tank once you reach the Stormblood levels. The amount of self healing and cooldowns blows everything else out of the water. Paladin is just in a really good spot overall and is imo one of the most interesting tank to play nowadays. Still boring as fuck getting there though, but it's a solid tank. If you want to play something else on your own though, anything's fine really, don't need the other 2 tanks, it's just easier to gear obviously since they shared gear other than weapons.
D) You have to do normal mode primals for a lot of the story. Normal mode primals are really easy though, and if you can get in a nice guild, anyone can just come and carry you through them easily. But in general people still do the old content since it's part of roulettes, you just queue for it and you should find people decently fast, especially with tank+healer. You might have to wait sometimes but can do side content and stuff, or as I said if you find someone to help you can just go 3man. Most normal mode primals can be soloed by a 70 until the Stormblood ones. Could also make a PF(party finder) asking for a 70 to help clear it quick.
E) No idea about buying gold but pretty sure they only punish the sellers not the buyers. You do get 1m for a fresh account if the promotion thing is still running, which makes money a non factor until you decide to level crafting/buy a house though. Although having a bunch of money would let you buy glamour gear so she can play a big part of the endgame FFXIV, dressup.
As for the leveling pots, lvl 60 doesn't really make you that strong. You could level it to 70 but that'd involve doing all the story quests first, which then means you wouldn't be doing them with her. You can still help her with some stuff but not everything iirc so might not be that great in that regard.
Generally speaking, FFXIV community is pretty nice towards newcomers(just might not want to say it's your GF to not get the creeps pming her stupid shit, like "eh we're new" rather than "she's new"), if you just point out that she's new most people will be pretty accepting if the dungeon takes a bit longer or they have to explain some things. It's probably good to not shelter her too much and try to do the content kinda how it's intended rather than have a 70 bulldoze through everything too.
And as I pointed out, there's still the Castrum fuckery at the end of the 2.0 storyline, if you level slowly you might not get there until the patch, but might be worth waiting a couple of weeks for that fix as it tends to dampen people's opinion of the game pretty quick when they reach that, especially since it's the most epic moment of the story that the whole thing had be building to(until you get to heavensward when shit get real good). 4.2 will be released either on the 23rd or the 30th(probably 30th).