No. If you're only going to be in the United States temporarily your International Driving Permit (IDP) will be sufficient for operating a motor vehicle. If you plan on becoming a resident you would...
Sometimes. A US citizen can marry anybody he/she likes. If a US citizen marries a UK citizen, that alone will not entitle him/her to UK citizenship. The US Citizen will stand to lose the US...
Why not?
There's plenty of examples of road trips both ways...
Prepare, plan and then plan again. Add another 50% at least to your budget, check all yr visa's and papers for every country you may...