1) With the charger mounted directly above the battery (so circuit length <5'?) redarc recommends 6AWG for 25A? That
is overkill. Or are you asking about wire to the solar panel? I'd use the fuse they recommend, they're not that big and you can get in-line MIDI fuse holders.
2) I think 10 AWG is the most common I've seen used for the fridge, I ran 8 AWG because I had it. The pre-made ARB and Dometic wiring use 10 AWG for example.
6) I was not happy when I tried to fit 8 AWG wires in the windshield trim, I ended up going through the rear instead.
7) they make through panel connectors you can use, I have a place to plug in 120V for onboard charging but you could use a similar spot for solar. I have no idea what wire gauge this is so I'm not recommending this product specifically, but this is the type of socket I am talking about
:
https://www.amazon.com/dp/B0031BOTFC..._eiHiEbRQ7QJK2
and here is my suggestion for where you could install it (ignore the red circle i was showing the antenna mount). The plastic panel pops right off and you can drill through it, it's cheap to replace if you change your mind later:
https://i.imgur.com/B3K4UK6.jpg