To unblock devmode developers, you should be able to:
$ sudo rm -f /etc/udev/rules.d/70-snap.yoursnap.rules
$ sudo udevadm control --reload-rules
$ sudo udevadm trigger
$ sudo sh -c "echo 'a *:* rwm' > /sys/fs/cgroup/devices/snap.yoursnap.yourcmd/devices.allow"
with the above commands, the device cgroup is disabled and the devmode snap should no longer be blocked. Importantly, snapd may regenerate the policy in various situations (eg, if you reboot, install, refresh snaps), so you might have to redo the above.
Note, since this is a devmode snap, the devmode snap would be allowed to do all of the above.