1) Is it easy to get to and from the mountain without renting a car? Can a US citizen even rent a car in Canada?
As Jed said, if you are in the Village/Creekside/Upper village you can easily walk to a lift. If you are staying in the upper village and want to ride Whistler and not have to walk, there is a free shuttle, a loop from the Village down to Marketplace to the Upper Village (I'm pretty sure never actually caught it) back to the village. If you aren't in walking distance a bus will cost you $2.50 each way, be prepared the busses (other then the shuttle) can get just as busy as waiting for a lift line.
2) Is mid-January a good time to go? Is there a high probability of getting some pow days?
It's Whistler the forecast changes every 30 minutes, compared to last season its been a really slow opening so lets hope for some good dumps later on
3) Are there any good shops in town to rent demo boards?
Depends what you are after, most of the shops get new gear every year. I always use affinity to rent for friends because they are the cheapest in town and have some good stock. Plus there's a demo tent up on the hill, I've never been in there so not exactly sure what boards they demo out
4) How is the nightlife?
pretty much every club has its own night in whistler, so speak to the front desk and see where they recommend for the night. They should also have some cards to get free cover for clubs like Tommy Africas, Moe Joes, and Maxx Fish