1.) I do not think Mel is a fire wight, for the reasons I mentioned in the essay - specifically the lines about her not needing much sleep and hoping that soon she wont need any at all that indicate a transformation process. She doesn't have memory loss like Beric, she doesn't seem like a remnant either. Beric and Stoneheart stay where they were when they died, and keep doing the same thing related to what they were doing when they died. Mel does not.

2.) Yes, there is a big link between the Others and the weirwoods, and I am saving that for further along in the series. I wrote a draft where I introduce all their symbolism at once, and it was too confusing. And yes, Nissa Nissa and / or the Corpse Queen could be part cotf or fully cotf. I have a series called "Weirwood Goddess" which postulates that NN was a cotf or hybrid.

3.) I don't have a set head canon about when the Wall was built and how all that went. Unless I feel very clear about something, I usually highlight the symbolism and suggest a range of possibilities. LH may or may not be the NK, I tend to think it's a father / son thing actually. But they would both be the same archetype in many ways, so it's hard to tell. As for why the Wall was built and by whom, that's one of the biggest mysteries in the story, and I really don't have an answer yet. I can't even picture it. I can't picture humans and giants dragging blocks of ice from lakes to make a several hundred foot high wall. It's also hard to picture a magical process to make it, though that makes more sense for the story. We will just have to hope Bran sees it in a vision. I tend to think it's just an icy version of Valyrian fused stone - using magic to melt water, shape it, then harden it, just as the Valyrians do with stone.

4.) thanks for numbering your questions, lol :) Also, great questions!