The shocks ARE replaceable with several conventional shocks, no problem at all. Bilstein, Ranchos come to mind. There's also a guy (Bruce something) that rebuilds the OEM ones for about $500 total.
I can't think of a single owner here that has put smaller tires on...I think somebody said 255/55-18s were a tad smaller, nothing significant, and not going to really affect MPG as the OEMs weighed 34 lbs, and just about anything else weighs more than that.
T4B addressed the releasable hubs.
Think you really need to see one in person soon, so you can pull all this info together in your head. I will mention again, if MPG is a big concern, this is NOT the vehicle for you. You'll just be frustrated.