Because damage remains marked on a creature until it's removed as the turn ends, nonlethal damage dealt to Golgari Grave-Troll may become lethal if you remove +1/+1 counters from it during that turn. In this case, it dies before you can resolve the activated ability that will regenerate it.

If you return Golgari Grave-Troll from your graveyard directly to the battlefield, its first ability counts itself.

Dredge can replace any card draw, not only the one during your draw step.

One card draw can't be replaced by multiple dredge abilities.

You can't attempt to use a dredge ability if you don't have enough cards in your library.

If you're drawing multiple cards, each draw is performed one at a time. For example, if you're instructed to draw two cards and you replace the first draw with a dredge ability, another card with a dredge ability (including one that was milled by the first dredge ability) may be used to replace the second draw.

If an effect puts a card into your hand without specifically using the word "draw," you're not drawing a card. Dredge can't replace this event.

Once you've announced that you're applying a card's dredge ability to replace a draw, players can't take any actions until you've put that card into your hand and milled cards.