For #3 see if there is some plastic drawer/tote/organizer thing that fits snugly. Like hit up a store like Canadian Tire, home Depot, etc and see if any half decent off the shelf sort of thing not designed to be that back seat organizer ends up being just the thing.
I've seen it more in commercial vans, and don't have examples in mind, but guys routinely come across something random that just works.
Like I have a Stanley 3 section rolling tool box that fits in the gap between my divider and shelving rack on my T250 for work. I throw my kneeling pad behind it and it is snug and doesn't move and is put of the way.
Cheaper and more handy than building an odd sized shelf, and is mobile.