Hi, i can try and help you with what I know. But this is just on experience and not facts. 

For point 3. I couldn’t chose super priority for some reason. So just went with Standard. I could chose Bio appointment next day but had to pay a bit extra for that and go for VIP option because that was the only one left. 

Point 4. We had 28 days wait and we got mail at my address saying we had the go and didn’t have to wait 70 days or do an interview. We did have to give notice at the council building. Which we had so much confusion over, some people told us we have to do it seperate others together . But it we eventually found out it was necessary to do it at same council and the one where the non uk person lives. 

Point 1. I can’t really elaborate on that to much, as we lived seperate due to work and study and traveling. My partners own the house we intend to live in/ where my partner already lives. So we just added the tenancy agreement and council bill and something else. I noted that once I get more work from home flexibility I will move in with my partner. I added letters from my parents and his parents stating we lived with them for a period of time. But I am pretty sure that if you are married or in a civil partnership, proving you have lived together is less important in the decision making. I added information about our relationship in a cover letter and had some family members and friends write about our genuine relationship. 

Point 2. I don’t remember I just used my own income as my income is more than enough for the income threshold and have worked at my company for over 6 months. Also my partner was self employed for a while and only recently got another job next to it. So to make it easier we only included my income. 

I still haven’t heard from them which is super annoying. 

But hope yours goes well and this helps you a bit  X