Lorkhans heart wasn't destroyed if I remember correctly, simply faced out. and the enchantments that bound it and made it accessible were destroyed.
Perhaps. Besides, the Heart was covered well enough in Morrowind, if Bethesda reintroduces it, I think it would be safe to say they are running out of ideas.
I didn't notice this before, but the quest header looks kind of like some tentacles. And if I'm remembering this correctly, Hermaeus Mora is supposedly a pile of tentacles...
It's probably safe to say that he will play a big role in the DLC
Considering Apocrypha is in, he/she/it probably will. Besides, incase you didn't know, the Benthic Zone is the lowest ecological zone in a body of water, so it would be very likely that Benthic Lurkers will be underwater enemies. Plus the quest header looking like a tentacled entity as you pointed out, I think its also safe to say there will be some very prominent underwater themes.