Hmm. Really the answer to all of them is a matter of opinion. Here's mine:

1. I'd say a good 50-60 party should have a decent chance, but what magic you junction would certainly factor into it.

2. Lionheart is pretty much essential because of Squall's level. This is because an enemy's HP and level are cumulative with his level, so at 91, she'll be pretty tough. It's not too hard to get Lionheart normally but... if you haven't got any Energy Crystals (for your pulse ammo) then it will be very very tough to make. (I'm not saying it's a disaster if you don't have it, but it would really help)

3. I don't personally use GF's, so I wouldn't say you'd need them. I doubt you'd be able to get Eden now (as you'll have missed it on the seven sub-bosses and Ultima is a struggle at any level (you'd really need Lionheart for THAT one...)) but the first level of the Deep-Sea Research Center you shouldn't struggle, and Bahamut should be a breeze.

4. All depends on where you level. If you can find the Island Closest To Heaven/Hell tolerable then a few hours should be all you need.