I'm only a week into the game so forgive my lack of knowledge regarding the game, lol.
Killing meph and lillith on t12, whats the perks for me?
pit runs, is that mad exp or what?
Lair bosses I get, its more loot the higher teir right?
I'll answer these for you
T12 Lilith and Meph are seasonal achievements, both give you a bunch of stuff because they are the seasonal rank 7 and 8 "capstone" achievement, as well as dropping good loot including likely a few mythics.
Pit runs are good xp if you're low in paragon, and you'll get some loot, but the real benefit is leveling your glyphs quickly
Correct on lair bosses.