@alexmurray, sorry, I did not tell you that I investigated the mode where the CUPS Snap and the system’s CUPS run in parallel, in preparation to implement the plans described here.
If you have no classic CUPS running and install the CUPS Snap, it creates the socket at the standard position, /run/cups/cups.sock, only if there is a classic CUPS running, the socket is created at /var/snap/cups/common/run/cups.sock.
To determine which socket the CUPS Snap is currently using, run
cups.lpstat -H