(I think the original author of these deleted their account, so this is mostly just replying in case others are interested as well)
1) It took me about 6 months, so probably a total of about 8 person months
2) I would guess about another 3-4 months or so to get to an acceptable level, and probably a year or two until it is a really full-featured discussion platform that can compete with the modern frameworks on a lot of core features. Though I expect this community will benefit much much earlier from it.
3) It seemed really really hard to fix the problems with the old LW. All the technology is really old, webdevelopment moves very quickly, and the codebase was not a great codebase. My read was about a year to get to a solid state, but then with almost no future-proofing and ability to expand on that.
4) I've spend about 1.5 months with a bit more than 60% of my time looking at various competitors and trying to build minimum prototypes with them. A full comparison of why the other frameworks didn't make as much sense is a bit beyond the scope of this comment, but we definitely spend a good bit of effort on evaluating the competition.
I thought that instead of everyone having to create a separate post for their first impressions, it would be more convenient to create a single post for this discussion. I'll post my own here soon.