Each creature with lifelink dealing combat damage causes a separate life gain event. For example, if two creatures you control with lifelink deal combat damage at the same time, a "whenever you gain life" ability will trigger twice. However, if a single creature you control with lifelink deals combat damage to multiple creatures, players, and/or planeswalkers at the same time (perhaps because it has trample or was blocked by more than one creature), the ability will trigger only once.

X is the power the creature had when it was on the battlefield, not the power it has in the graveyard.

When Dina's first ability resolves, each opponent loses only 1 life, no matter how much life was gained.

Dina's first ability triggers only once for each life gain event, no matter how much life was gained.