Basically all pre-SoA content can be soloed as a ilvl119 player. Some jobs will have an easier time of getting things done though, so you probably should consider leveling/gearing at least one melee job as well. Personally, I suggest THF since Treasure Hunter can be an enormous time/frustration saver and it does good damage even in endgame content. First job will always takes the longest (due to no merits, level break quests, missions, etc.) but for 2nd+, expect somewhere in the 8hr range for 1-99.

1. Not hard. You can still do the mini-trials if you don't want to wait til you are high enough leveled to take on the standard primes.
2. Nope. At least not in the standard primes.. Never done the mini-trials myself, so unsure about those, but expect that is also a nope.
3. Burn a melee job all the way up to 99, get at least some sparks gear, then go splatter the primes. If you go this route, you could also farm up a bit of gear for smn in advance using the melee job.. It's nice having Carbuncle Mitts waiting when your SMN dings 20. ^^
4. Level grinding is insanely fast compared to the old days. As long as you do your RoV missions ASAP to amass at least 6x Rhapsody KIs as possible, you are looking at a baseline exp+180%. I am not exaggerating when I said to expect ~8hrs for the 1-99 grind (soloing with trusts). I've leveled all jobs to 99 and my record for 1-99 so far is 5hr49min, but I had already capped out all merits, had all ROV KIs, used COR rolls, and made sure that I always had an exp ring buff active. It really does not matter much if you choose to level a non-SMN related job first since we are no longer looking at large time investments to do so. Most of the SMNs that I know usually have RDM/SCH/WHM as subjobs if that helps.

I suggest you level THF as your first job, take it 99, use it to get missions (especially ROV) and merits done, then work on SMN and subjobs. Don't forget you will need to get a Carbuncle's Ruby first in order to unlock SMN, and it's still a very lousy droprate.. TH will help with that. Unless you are coming back to an established support network (same ls/friends still around and willing to help you,) this would probably be your best bet, IMO.