CO2 under pressure RAISES the melting point of ice (by about 10 degree at 5 bars [atmospheres] pressure, and 20 degrees at 100 bars), by encouraging the formation of CO2-clathrate, an alternate ice structure with both CO2 and H2O.

Clathrate seems more or less immune to the effects of salts on reducing melting point.

I'm not sure about the effect on boiling point of CO2 under pressure, although the solubility of CO2 in water decreases with increasing temperature from a max of ~7% at 0 C, so the effect is probably minor.

The effect of pressure, per se, is to increase the boiling point in a well-understood way for water until 374 C and 218 bars is reached when it becomes impossible to liquefy the water (This is "supercritical" steam).