1. 20" are fine so far on 3000 miles, unsure long term. Gravel was limited to side of road.
only warning is 20" are summer only. Mistake I made, I will have to buy winter tires and 19" wheels before fall.
2. Iowa allows driving 30 days on paperwork, not sure if it is entirely legal, but think it is fine. CA gave me a 1 trip permit. I wasn't ever stopped.
3. I drove interstate most of the way back for convenience, not a lot of side roads in this area. I stick to backroads locally when running gas, since limits in Iowa are 55 mph on back roads. Meaning I have no issue flowing with traffic at 60.
GPS in my model (upgraded Nav) has route options, if you select ECO PRO routing it will pick back roads, watch it though, sometimes it picks much longer time routes, especially in urban areas. Like driving on surface streets in traffic. Maybe use Phone to check. On Google Maps you can get ideas for better routes saying you are a bicycle (watch that they are roads and not bike trails though, I do it to get alternate road ideas).