Since the last time I played the quest, I learned that there was an alternative to taking sap from the Eldergleam to repair the Gildergreen. But instead, I get a sapling.

Will I see this sapling be planted somewhere? Or will there still be a dead Gildergreen outside?

If you go the sapling root route you will be able to see if after completing the quest.

I see what you did there...

I'm seeing blooming on the Gildergreen now and I delivered the sapling too but I don't remember if it was completely bare at the beginning of the game. Anyone know?

Yes, it was.