Multiple instances of lifelink on the same creature are redundant.

If an effect copies a spell with an addendum ability while it's on the stack, the copy wasn't cast at all, so you won't get the addendum bonus.

Gaining vigilance any time after the moment you choose to attack with a creature won't cause it to become untapped, and losing vigilance after that time won't cause it to become tapped.

If Sentinel's Mark enters the battlefield without being cast, the addendum ability won't trigger, even if it's your main phase.