I haven't been in the hut in winter, but I think it is generally opened all year round. Usually there is noone inside. When empty it is closed with a wire, so no problem with enetering (maybe you'll have to use some energy to get rid of snow'n'ice blocking access to the door though).
Yes, you right. About so called 'winter room'. While I was in Hornli Hut in winter there was an opened room, with 2 stoves, 15 beds and plenty of food left by climbers in summer season. I thought how is it in Carrel because I was there also only during the summer.
In Carrel there are stoves in the main room and plenty of food, I think it is same both in winter and summer, gas valve is inside, but I think there is another, main valve hidden outside (so told me a Romanian friend). Sleeping room may be closed, don't know, but you can use the benches in the main room, there are some blankets also.