Hi Fantastical,
-18 C would be in the freezer. I would think that would turn it solid ice, so you'll need to thaw it out to use it. Not sure what a repeated freeze/thaw sequence would do to it? I'm sure you'll get some expert advice though.
As for the whey, you could make ice cubes and then use them as starter (2 OR 3 standard sized ice cubes, aroudn 25 ml each, for a 10 L make probably would do it). The would contain your blue mold in it, so you would have to use them for blues only, but it should work ok.
- Jeff