We've mentioned that possibly Molly went back and possibly ended the Ice Age, but it seems to be assumed that she did so in the persona of Mab. I think it more likely that the Summer Queen would be the one to end the Ice Age. We haven't seen Titiana directly in the story so far and I think that's part of the big reveal.

Molly is Titiana!

Why is Winter so interested in Molly? They know she will become (had become) the Summer Queen through time travel and a rite of ascension. Summer is moving around mostly in the background, which is very much Molly's MO. Sure, the Queens are renown for their sneakiness and subtlety, but so far Mab is the one who has been directly involved with things. Summer is so sneaky that no one has seen the Queen yet!

Why is Summer being so subtle? Because the Summer Queen is Molly and she doesn't want to interfere with things that are going to bring about her own time traveling and ascension. Also because she thinks Harry might start to recognize things if she gets too close, and so the Summer Queen stays away from Harry.

It could also be Molly/Titiana's voice Harry hears at the end of Changes. If Summer is behind the assassination (there are a variety of reasons possible which don't involve Harry actually becoming permanently dead without hope of resurrection/restoration) it might be possible that it is Molly/Titianna really is keeping a close eye on things. It all fits!!!

*cackling madly*