Hi and welcome to HT!

1. Yes you can get your insurance in the UK. Here are a couple of places you could try.

http://www.icarhireinsurance.com

http://www.insurance4carhire.com

2. No, and it is very expensive to buy at the desk.
3. Not usually. You may be cheaper buying US maps for your own sat nav or using your smartphone

You can also book your car hire here with all insurances included. Try netflights or http://www.carhire3000.com

If you pay for your insurance in the UK, don't let the desk clerk try to upsell you or tell you your insurance won't cover you.

For satnav head to the nearest WalMart or Target (about three miles away) and buy one there. Bring it home and advertise it on Ebay. Satnav is usually referred to as GPS in the States,

Some rental companies do not accept insurance other than their own so best check before you go.