General

1. Depends on your range, preferences and the type of boss you're facing.
Generally Rapid Fire works for all bosses now that AB has been nerfed.
it is 120ms of 270% damage vs miles' 300%damage every 180ms.
Arrow blaster is good if you want to avoid touching bosses and like to snipe them from far away though its 163%casted+112%planted at 120ms.
Final cut is only useful for daily bosses and for players that have low range to begin with.
Use FC -> Switch it out for another skill -> gain 30sec of FC with new skill / 60sec with miles.

2. You are right, canon bazooka is the best mobbing skill for phantoms, especially for soloing only.
For things like kills count- Kritias: ArrowB + Canon is the best way to farm kills for quests with good enough range.
Training wise, Bazooka is for soloing [700krange+] or use Showdown if you have a party so you can just worry about a single corner of the map.
Broadside is the typical botter choice. At best you'll only get around 2~4 kills per second, which is already less than half the mobs showdown/bazooka gains.

3.
Haste if you dont have max speed/jump, otherwise Darksight.
Rage if you don't have the damage [lol] and heal for pot-lock bosses.
Cross-surge for sure, but holysymbol for training if you can.
as a party mule : advanceblessing/sharpeyes/speedinfusion are useful
but 4th job buffs aren't needed since you'll need one slot for rapidfire and another for showdown/bazooka.

4. Guilds/Alliances [search people's guilds for online members],
mainly Ch1 henesys/leafre/DiPQ/SDH/FDH.
especially during peak hours of 4pmEST+.

1) Platter + Mille is very effective. Another option is to use rapid fire or hurricane. There are very significant differences between the two, despite having the same damage. For one, rapid fire has a shorter cast animation, so it has less delay. And for some reason, you can't use auto judgement draw with hurricane. So unless you want the capability to jump backwards while attacking, rf is preferable. Final Cut + Mille is good, but i don't think it's necessary to steal fc. I don't know the math, but i feel like the extra cards you summon from using rf/hurricane makes up for the extra damage from using final cut+mille.

2) Broadside is a very strong hold down skill, so it's good for training. Bazooka is pretty slow, but the reason it's used is the range of the ability. Being able to hit half of SDH without moving is quite good.

3) Cross Surge, Holy Symbol, Rage, Hyper Body, Haste (for a pt), Speed Infusion, Sharp Eyes

4) Since they removed the ability to steal from FM, it's become a lot harder. Playing from Windia, i don't really have too many issues, but try DIPQ or the PQ area.

1) My personal opinion is Magic Armor+Rage+Cross Surge+Hurricane. In order: Magic Armor is just a little extra defense, No one really needs haste now, anyways. Rage: Extra attack and party support. Typical. Cross Surge: It close enough to doubles your range. It throws me into the 190k range which is awesome. Hurricane: Mille isn't all that great for bossing so your choices are Hurricane and Rapid Fire. There's a definite difference between the two but I can't really explain it right now.
2) Magic Armor+Rage+HS/Cross Surge+Speed Infusion. Use Mille Aguilles and Penombre to get around quickly. Cross surge if your damage is too low and HS for grinding. Speed Infusion's a personal favorite but I guess there are better choices. I personally would recommend Blade Fury if you really needed a different mobbing skill but try to stick to Mille and Penombre.
3)Best Buffs: Cross Surge, Combat Orders, Holy Symbol, Rage, Hyper Body, Haste, Magic Armor, Speed Infusion, Sharp Eyes. Admittedly you don't need sharp eyes once you get far enough because your critical rate should be 85% to 100% so it's only great in a party.
4.Galicia isn't the best place to steal skills nowadays. My best suggestion is the PQ lobby but that's also a little shaky.