1> Is there a way to find out which boss attacks are melee / ranged or which are AOE ?
You're confusing the type of the attack (melee/ranged v. Force/Tech) with the number of targets (AoE v. ST). An AoE attack is any attack that can hit more than one target. An ST attack can only hit a single target. The type of attack is an arbitrary distinction that simply determines what mitigation mechanisms can be applied to it (M/R attacks can use defense chance and can be shielded; F/T attacks use resistance chance and cannot be shielded).
Basically, should we spec for AOE damage reduction talents or not?
Yes, you should. It's important to mention that 30% DR is factored into your mitigation, but I'll get to that later.
2> Similarly, is the railshot from the 2 giant Kephess droids ranged or tech? I used pacify on them, not sure if it helps or not
Rail Shot is a ranged attack, but, on SM and HM, it shouldn't be an issue. If everyone in the group is stacking inside of the pulsar droids like they *should* be (re: their listed ranged is 0.0m), the boss won't rail shot anyone on HM. If a single person is outside of this range, however, the boss will target a random person with rail shot; as such, a single person screwing up by not standing in close enough can end up killing an entirely different person (and often does).
Pacify should't work because it doesn't work on Operations bosses, which I believe the Pulsar Droids count as (else it would make it really easy to cheese the fight by using Pacify/Diversion).
4> I am not clear on the difference between Kephess's knockback and this Breath cast. Knockback applies the Touch, causing the tank to turn black and start dropping circles, so why cast the Breath ? Seems redundant
You're getting it wrong. The KB applies Touch of the Masters, which means that, until that debuff falls off, any time the tank takes damage, the tank dies. Breath of the Masters applies the debuff that turns you black and causes you to drop patches of bad. The separation of the two abilities forces the tank to actually keep the hell away from Kephess until the Breath begins casting, which gives the other tank a easily visible taunt window (since it's easier to see a boss casting than it is to see a debuff on your buddy tank).
5> And finally, does damage reduction stack? So if I take the sentinel talent defensive roll, plus pop both camo and rebuke during Gift of the Masters, I should take 0.7 * 0.5 * 0.8 = 0.28 of the damage from the Gift AOE.
Yes, damage reduction stacks, but not in the way you think it does.
First off, you need to remember that Force Camo only lasts 4 seconds and ends immediately if you make an attack; if you're using before Gift of the Masters in preparation for it, chances are, it's either falling off thanks to duration or getting negating by you making an attack before them. Rebuke isn't much better since it falls off if you haven't been attacked in 6 seconds. In all likelihood, you don't have one or more of those buffs.
Secondly, it's important to note that while all damage reduction is percentage based, not all damage reduction effects are separate. Rebuke directly adds to your listed DR: as a Sentinel, you should have roughly 25% K/E DR and 10% I/E DR passively; with Rebuke active, it'll bring you up to 45% K/E DR and 30% I/E DR (GotM is kinetic damage). The AoE dam redux talents and most of the other situational damage reduction talents (30% less damage taken while CCd, etc.; afaik, Warding Call and Saber Ward's F/T DR effect are the only CD based effects that use the same mechanism) that reduce the total incoming damage by a flat percentage rather than adding to an existing damage reduction mechanism.
GotM does something like 6000 base damage per tick so, with 25% baseline K/E DR, with no other sources of damage reduction (no Rebuke, no AoE dam redux, etc.), you would take 4500 damage per tick. Throw in Rebuke, it goes down to 3300 per tick. Add in the AoE dam redux, and it should go down to 2310. If Force Camo is actually active, it would go down to 1155. Keep in mind, it ticks every second so the damage ramps up *very* quickly, especially since Force Cloak isn't likely to last.
Rather than trying to survive the damage, your best bet is to simply try to get everyone out of there asap. If you've got a sent, all anyone should need is the Transcendence buff, which should be more than enough to get everyone out before they die even if they're not using any CDs.