Agreed, this is not nearly plug and play.
Yes.
No. Not if there is no parasitic draw, i.e. alarm system, solar controller bluetooth module, dodgy ground, etc.
Given your use case, no. Not a bad idea. 👍
Given your intended use, this is all good. If the alt upgrade to the 60 is nearly plug n play, this too should be fine.
No not at all.
Umm, now here...
...If you're going to bother to wire to the rear, just wire it to plug into the GZ.
Then plug everything else into the GZ.
At most, this is...