Is it a good idea to install a battery disconnect switch for storage ?
Yes, it's a good idea. My battery has a wing-nut on the terminal. Easy connect and disconnect
2) How important to have two batteries for fiver ? Our 2010 Outback has one. Am concerned could get stuck unable to retrieve slides.
Very important if you DRY CAMP. If you don't, I don't see the need for two batteries, unless you've added a few more 12 volt powered items.
(4) Do have a wrench to manually crank em in but don't look forward to that job.
Why, It's not all that hard to do and it's not like you'll be doing it constantly.
Could install a second battery or just carry a fully charged backup. Which would be better ?
I believe installing a second 12 volt battery would be better. This way, it will be charged continuously when connected to shore power.
By the way, what happened to question # 3 ?