Because damage remains marked on a creature until it's removed as the turn ends, nonlethal damage dealt to Honor Troll may become lethal if your life total drops below 25 during that turn.

The middle ability of Honor Troll applies just once to each life-gaining event, whether it's 1 life from Impassioned Orator or 4 life from Dawning Angel. If you gain an amount of life "for each" of something or "equal to the number" of something, that life is gained as one event and the ability of Honor Troll applies only once.

Each creature with lifelink dealing combat damage causes a separate life-gaining event. For example, if two creatures you control with lifelink deal combat damage at the same time, Honor Troll's second ability will apply 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 apply only once.

If you control two Honor Trolls and you would gain life, you gain that much life plus 2. A third Honor Troll has you gain that much life plus 3, and so on.

If Honor Troll is dealt damage and your life total is increased above 25 before state-based actions are performed (most likely because Honor Troll is dealt combat damage at the same time that a creature you control with lifelink deals combat damage), Honor Troll's last ability applies before checking whether it's destroyed for having lethal damage marked on it.

In a Two-Headed Giant game, Honor Troll's last ability applies as long as your team's life total is 25 or more.

In a Two-Headed Giant game, life gained by your teammate won't cause Honor Troll's second ability to apply, even though it will cause your team's life total to increase.

In multiplayer formats where your life total doesn't start at 20, Honor Troll's last ability still applies if your life total is 25 or more.