1) The only weather sealing issue I've heard talk of, is that if you plan on using it in rain, keep the hotshoe contacts covered. (don't lose the little plastic insert, in other words) If water pools in the hotshoe, connecting the contacts, I've heard it can cause problems. But I haven't experienced that personally, and I have personally used it, uncovered, in light rain without any issues.
2) Sony does have their own version of CPS, but you have to own 2 bodies, and I only own one, so I haven't looked into that much. https://esupport.sony.com/info/1523/US/EN
3) Who can say
4) I'm guessing you already know you can use Canon lenses (adapted), otherwise you probably wouldn't even be considering it. But yeah, of course you can. The tiny flange distance means you can use practically any lens that covers the frame, and has an adapter available for it. Not all adapters are created equal though... but that's a whole other topic
A lot of people are indeed loving it. Other people try it and then decide it's not for them. So that's a tough call that only you can answer. I came over from Canon, original plan was to use the A7Rii as a high res studio cam, and keep using my 5D3 for other scenarios. It worked better than I thought it would, in all my typical shooting scenarios, and I ended up selling my 5D3 because I just wasn't using it anymore. A friend of mine also bought an A7Rii around the same time, and decided he didn't like it, and returned it. Only you can say if it's for you... the crappy part is that you probably won't know until you've had time to seriously put one to work. I'd suggest only buying from a vendor that has a no questions asked return policy, and test the hell out of it within that window. If you don't dig it, return it. If you do, dandy.
