The answer to this obviously depends on the capabilities of GPU's "single head". It is completely possible that a DP 2.1 UHBR20 GPU may in fact be able to run DSR on a 4K/240Hz DSC display.
Access to (DL)DSR depends more on the monitor's electronics than its use of DSC. If the manufacturer wants it they can make DSR work with DSC.
Sure but such cards a) exist already in provis space and b) are coming to gaming space fairly soon...