Installing leisure electronic setup/advice

[jgm0]

Hey all, I'm new here and this is my first time posting. I just got myself an ex-AA van and am really excited to be part of the community and get to work on my van. I am feeling a little overwhelmed to have just spent a lot on everything (I think) I need to setup the electrics in my T6. I've only dabbled with basic 12v before. I still haven't decided where to install everything! I was hoping to get it under one of the seats but I want a swivel on the double passenger and I fear the setup won't fit under the drivers seat. So, looking for some reassurance and have a few questions:

1. Is it possible to install electrics under the passenger double swivel? My guess is not since the whole base swivels
2. Where is the best place/most common place to install the electric setup?
3. I have tailgate model, is it possible to run the solar panel cabling through the tailgate wiring to save drilling the roof? (I plan to mount the solar to my roof rack)
4. Any experts here? Can someone look over my diagram and let me know if there are any issues?
5. My exAA van came with an Aux battery under the seat for the split charge relay, I have read all the posts on this but none the wiser. Can I utilise some of the cabling and is there any guides on removing it?

Any wise words, reassurance or tips would be greatly appreciated! Cheers.