- Stacking no, but mastery still affects Divine Aegis. You have a pretty high crit chance with your single-target heals, and of course PoH will proc a DA on every single target, so Mastery is still a pretty good stat despite the fact that you can't spam PW:S anymore.
Keep in mind that DA caps at 20% of your max health, and is also reliant on your crit and haste levels (more crit and more haste = more frequent DA procs), so stacking nothing but mastery is a bad idea. However, it certainly has its place - personally when I play Disc, I reforge for a mix of crit haste and mastery, as they all complement each other quite nicely.
- PW:S is basically for tanks only, although if there is a predictable damage spike on a single individual (like Magmatron's fire beam, whatever it's called) then it certainly could be used there. I would avoid overusing it on the raid as like you mentioned it will deplete your mana pool very quickly, but it is still a very potent tool and an important part of Disc Priestage.
One of the best ways to keep your mana bar full is to get a timer for Rapture procs, and make sure you cast your shield on a target who is about to take damage juuust as it comes off of cooldown. This is something I'm still mastering, but it's a big help.
- Because they reworked PW:Barrier to be a flat damage reduction (like Pain Suppression) it is not affected by Mastery.
- Probably not. I still believe that the best way to gear is a Disc Priest is moderation in all your secondary stats, perhaps with a weight on Crit if you're mainly single-target healing but even then, mastery and haste are still good.
edit: also, the word is "absorption" - still, very good English for someone who isn't a native speaker