Knockdown says the creature is incapacitated until the start of its next action phase. Does this mean the one coming up or the one after? (a common usage in normal English, but less clear in a strict rule context)

I assume that means, as soon as it action phase starts during the round it is no longer incapacitated? The usefulness of this spell would be to ...knock down a flying creature after its action phase is incomplete so all of your melee creatures may attack it? This means it would not prevent it from taking its normal action when it comes time...

the priestess has an ability that lets her gain 1 life after casting a holy spell (incantation?)

If she is at maximum life does her life increase? Since it doesn't say "heal", it seems unclear...