[elhigh]

A1: There are battery isolators that will allow you to place one battery on a circuit separate from your backup power while still being in the charging loop. It's usually intended to protect an RV's starting battery while allowing the RV's charging system to top up household batteries, but I think it could work in this application.

It's not hard at all to find a switch that would allow you to treat the batteries as equals, A or B. Simply put the charging circuit on the batts in parallel so they charge equally but run them individually to the A and B sides of the load switch.

NOTE: Tying the batteries together in parallel would actually run your household loads (slightly) more than twice as long as running each individually.

A2: According to the owner's manual for the charge controller, it is a charger for the batteries. Why they insist on calling it a charge controller is a bit beyond me but if I had to guess I'd say it's because it's a bit more than just a charger.

A3: Like for A1. There are isolators that will allow you to keep your starting battery in the charging circuit along with your household batts, while keeping the starting battery isolated from household loads. It would be kept aside for vehicle purposes only. Or you can have it off on its own circuit and ne'er the twain do meet. Your call.

A4: Totally your call. Adding a cap means adding area, so if you are really careful at your design and build you could wind up with a net zero. It would give you the opportunity to carefully fair in your solar panels so it has its merits. Parking with an eye toward sun orientation will yield results but again, your call. Sometimes you just have to park inside the lines.

A5: My understanding is the Lithium-Ion batts don't need venting except in the case of catastrophic failure. That catastrophic failure mode can be VERY catastrophic, flames and whatnot, so it's not a trivial concern. You could consider a battery bank built up of salvaged 18650 (laptop) cells and have a significant amount of power storage in addition to an extremely flexible form factor. Something like that could be built into tucked-away packs that lived under the van where venting into the passenger space was no longer a concern.

A6: Windy Nation has a great rating, I wouldn't hesitate to trust a company with a rating that good.