Lodging in banff is relatively cheap compared to other resorts so you can definitely splurge on higher end accommodations.
The downtown is not huge it's only maybe 3 blocks by 4blocks or so, so it's fully walkable, but this depends if you picked accomodations in town town, or in the outskirt resorts.
Transportation to the big3 resorts via shuttles is very good. However you do need to plan for the bus schedule and the resorts are a distance away from town and an hour between the buses.
You do not need a car if you only are doing skiing and staying in town, but if you want to go do other activities other town a car makes it a lot easier. Keep in mind your daylight hours gonna be limited so you have to choose what you want to do.
Activities, realize it is Canada's first and biggest national park, so a multitude of outdoorsy activities or outdoorsy sights; + banff hotsprings , the Banff Fairmont or LL Fairmont, or going to KickingHorse which is feasible as a day trip.
Nightlife, I am not sure about, remember it is quite cold, so people are not really walking about as much as other ski areas, I was in early most nights to get the most of the day
Agree, snow is not prodigious at all compared to other, but the temperatures help maximize the minimal snowfall and keep it for awhile don't melt away, but chances are you don't even have full coverage yet.
Xmas/New Years typically is early season in most places anyway, it's always been a marketing ploy to pull the holidays in as part of "winter".
If you are an advanced or expert skier, the resorts are somewhat not very terrainy at first glance, you may need to seek out pockets and stashes that fit what kind of skiing you are looking for away from the main runs.