Good questions! I don't claim the last word on any of them but I like theorizing and Rebel is one of my favorites, so here are my thoughts:
1. MVP skills - All the ones you listed are good high damage skills. Dragon Tail supposedly doesn't miss which makes it a big contender to me. Slug Shot can certainly one-shot some MVPs when it lands. Binding Trap is really strong while the MVP has lots of HP left, the higher their current HP, the better it is.
Other options -
Crit-lock with Heat Barrel should be viable, best if you can kill the MVP during the 1 minute duration. I suspect crit is better than auto-attacking with Dual Action/Quick Draw Shot procs because right now it looks like Quick Draw Shot is very hard to proc at high aspd. If that's a bug and they fix it, then I'd say non-crit auto-attacking would be viable too for non-AGI UP MVPs.
Desperado or Fire Dance with cast delay reduction gear has great DPS potential if you're able to go toe-to-toe with the MVP.
2. Leveling options: Round Trip shouldn't change much from how it's used now, its viability won't change much. Other styles will be more viable, will have to see if any of these are more effective than Round Trip:
Shatter Storm has better damage mod than Round Trip, but has fixed/variable cast, longer cooldown, and smaller (7x7) AoE when maxed. Should be able to get higher base atk with a shotgun than a gatling so if you can one-shot with Shatter Storm and not with Round Trip I think it could be better?
Desperado/Fire Dance are both able to take advantage of delay reduction gear. Leveling with these with cast delay reduction gear would be similar to GX with Rolling Cutter. Very mobile leveling like this with Fallen Angel too (so far it looks like Fallen Angel only works with revolvers). Fire Dance starts with .5 second delay so with HWHB, NWS, FLPx2 (with EA 1/2 set bonuses) you can get it down close to 1/4 second delay. Desperado is harder to get up to RC speed, strings or Kiel needed I think. But it does double damage for 2 seconds after casting Fallen Angel, so still a good opener.
I think Howling Mine would be kind of fun to level with by getting mobs to chase you and dropping a new mine on each mob you find, then when you've reached the three mine limit, get them to group up and blow them all away! But I don't know that it would actually be as effective as other methods.
Fire Rain has a 5 second cooldown but a very nice damage mod, might be able to one-shot earlier than the choices above if you are good at positioning mobs.
3. Best guns: It sounds like we aren't getting the new guns with the Awakening so it's just the stuff we already have access to. Pistols have the best variety of options with Crimson Revolver [2], Altair & Ares [1], and Vicious Mind Revolver [1] all looking pretty good. Other gun types only have vending machine options as far as I'm aware.
4. Job exp table changes: Don't know, get job 50 now!
5. Gun types that work with Fallen Angel: See above (2.). In Sakray, Fallen Angel is only working with revolvers.
Edited by Orthogon, 20 November 2017 - 05:39 PM.