The total power of the creatures you tap is checked only when you pay the cost. It doesn't matter if their power decreases, or if any of them have left the battlefield, by the time the ability resolves. Note that you may tap a Mossbridge Troll to help pay the cost of a different Mossbridge Troll's ability.

The first ability essentially means that Mossbridge Troll always has a regeneration shield. It retains that shield even if it's used.