Everyone's been correct.
You'll gain mana back for any actual healing received that is credited to someone else in the combat log. Think of each separate HoT tick as a very small healing spell cast on you by the Druid. Each one will give you mana returns. Lifebloom blooming will not, since the healing is credited to you. As always, you don't get any mana returned if there wasn't actually any healing done (over-healing doesn't actual heal you.. so you don't gain mana from that).
You also gain threat from being healed, though not directly. You actually gain threat by regenerating mana. Everyone gains threat by regenning mana (including casters when they're outside of the 5 second rule) only you tend to regen a TON more mana than anyone else. The amount of threat you gain, though small, is NOT negligible. If you check out a parse of a boss fight, you'll find that it's actually a fairly decent % of your total threat (we're not talking 10% here, but still not negligible). If you factor out the DPS and just look at the fight in terms of your healers and you, you can effectively say that healers generate less threat by healing a Paladin tank for this reason. While technically they generate the same threat as always, the mana that is returned to you generates threat and works to counter the threat that the healers generate.
And befitting to the mechanics by which a Paladin tanks, any mob that has "noticed" you, (ie. has you in it's threat table at some point) will build aggro on you from the mana returned through Spiritual Attunement, so it contributes to AoE threat generation as well.