1. I'd be terrified to run as a sole proprietor for anything more than getting started. I work on medical software, and if someone dies during a procedure - even if it's not my software's fault - I don't want my personal assets to be liable.
2. I started at 35%, and moved back once I saw what I was actually paying. Depends on your state in life, state within the US, and charitable giving state-ments.
3. Yep, 2 months is low. Just enough to weather a storm, but more = better.
4. Funnily enough, I'm switching to hourly billing for much of my work. But I think many freelancers (particularly US-based) don't even realize "day rates" are a thing - we're fixed on hourly pay, for some reason.