I disagree that you can't carve on a alpine oriented board with a soft setup. It's not the material that restricts you, but it's the rider himself. An alpine board with hardboots will only help you with a better carve (more control).
Icy conditions, take a relative soft flexing but stiff torsional wise! board, it will hold better than a very stiff one, and it will give you more comfort. If you have a soft set up which gives enough support, you can ride it on an alpine oriented board, no problem. I ride a POGO Longboard 175 with softies and also can race this one with my soft setup (I also have Vans boots with that BOA thing, stiff enough!). My soft set up is a rather supportive one. Racing Skiers have a problem to overtake me with that alpine oriented POGO. That Donek Axxes with a soft setup and that wide waist looks fine to me. A nice allrounder for some one with a bad knee. With that 24 cm waist you will have a nice platform for the more powdery days. Don't know how stiff that Prior ATV is, but looks like a good one too, I think with a radius of about 10m, it's a turny one for sure. The only downsite with a wider board s that the edge change will be a little less faster but you will also have more comfort on it. May be you have to wait a little and look in the classifieds for a good secondhander. There will be interesting boards dropping in when the first snow is there for sure. Seems that an allround alpine oriented carveboard will suit you best. So no Silberpfeil for sure (not good in softy snow conditions).