OK so the story is that service usually has the roof racks in stock but they can't tell me until Monday. So I'll either buy them and get them fitted during the week or just hire the skis down there. On the issue of snow chains, as an AWD vehicle the Tesla is apparently exempt from the requirement for snow chains. So far, so good.
But here's where it gets interesting. I was planning on using the mobile charger over the 5 nights of our stay. But apparently if the temperature is below -5 degrees, the mobile charger can only just keep the battery warm and won't be able to add any charge. So if you're parking outside, that isn't going to work. We'll be parking in a garage so that should mean it's OK, but there's obviously still a chance that it won't charge much. However there is a destination charger at the Alpine Hotel, as has been pointed out, and so that is a backup option.