Army units don't use any resources once you remove their buildings so I don't see much point in trying to get rid of the units. I'm pretty sure that if you order units to rest and destroy the building while the unit is inside, unit will disappear with the building. Never tested that, though.
1&2) Both suggestions are implemented in my mod
3) Or you can load a trade cart or a boat with resources and send it together with a bunch of settlers.