Your setup seems very well thought out!
I guess the idea with a dual battery setup (other than increased capacity) is to stop non-vehicle items from draining the vehicle batteries, which means the vehicle batteries should always be in good enough state to start and run the vehicle. If somehow the vehicle batteries do run down then your "reach around" method is perfectly adequate, adding in an automatic slave start feature will probably just be adding in more complexity unnecessarily.
I'm looking at potentially going for a MPPT & DC-DC charger combination unit for my installation. I'm in South-Africa so solar makes complete sense here, I just want to keep it as low as possible to the roof and haven't quite decided how to do that yet (wind resistance reasons). The dc-dc part is needed because I want to go for LiFePO4 batteries, mostly due to the weight savings and longer life even at deeper discharges.
A friend of mine has the same Waeco fridge as your newer one and he is very pleased with it. I like it too and have had many a cold beer from it in unpleasantly hot conditions.