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?
If you're staying somewhere in Whistler, there's plenty of buses. No need for a car to get around within Whistler.
Most places are walking distance to one of the lifts, or a 5 minute bus ride.
2) Is mid-January a good time to go? Is there a high probability of getting some pow days?
January is as good a time as any. We get plenty of snow during the winter months.
January gets an average of 7.5 feet of snow each year, which is about the same as December and more than February.
3) Are there any good shops in town to rent demo boards?
There's a lot of shops that rent demo boards. Most popular is probably Showcase Snowboards, which is owned by the mountain.
4) How is the nightlife?
Whistler has probably the best nightlife out of any ski town in N. America. Plenty of bars and clubs to choose from.