I'm afraid I have to question most of this, the sublimation temperature of CO2 is ~78.5oC and the liquid form at high pressure (over 5 bar) its a bit more complicated than just solid/liquid/gas you need to take pressure into account to, good to have a look at the phase diagram.
phasediagramco2.png
I doubt any domestic freezer is ever going to get cold enough to freeze a bottle of CO2 solid. The other point about embrittlement would be important if CO2 bottles were made out of mild steel, they aren't (well the disposable single use ones might be), to get approval in Australia as a CO2 pressure vessel they would have to be able to withstand repeated filling and emptying and the cooling that occurs when the bottle is vented (i.e. -78.5oC) which is very stressful for the bottle so they are made of a spring type of steel, designed for the job, or Aluminium that will actually get stronger as it gets cooler (to well under the temperatures we are talking about).
Unless your freezer is getting down to below -78.5oC I wouldn't be worried.
Mark