2. Since it takes a lot of time to stack up lifebloom, you get more overall healing in PvE for rolling it. You also can't exactly time when it will bloom, so consistently letting it bloom will waste time and result in over-healing. There isn't really a point to the bloom part of it, though the bloom tends to be more useful in PvP (where you get stunned and incapacitated more and the burst is nice when you can't do anything).
3. Unfortunately, the answer to this is: Until you end the fight without running out of mana. If you are always ending fights with 50% or more mana, you could probably live with less spirit (at least in raids where you get to stop and drink). If you aren't killing things because you went OOM and the tank died, you definitely need more. Otherwise, it's really about personal preference.