I retried with the black screen trick. The sx280 seems fully working with that, I'm able to switch play<->rec, shoot.
The M10 doesn't fully enter rec mode, shows playback screen but mode list gets populated. *If I half-press, it asks whether I want to disconnect. If I say no, it remains connected and nothing changes, otherwise connection breaks and cam enters real rec mode. Did not look for that variable yet.
*That happens to match what you described above.
Poking the variable (0x8adc, @FC341E6C in m10 101d) resolves the problems, play<->rec switch, shooting works.
Thanks. That matches what happens on my cams.
Poking the variable has some odd side effects on my cams: On both sx710 and g7x, a bunch of menu items in the Canon settings menu are set to defaults and greyed out. Most obvious is the "mute" option, which is turned of, but looking more closely a bunch of others are too, including "AF point zoom", date / time, and on G7X "Night display". Additionally, on g7x, some func menu settings seem to be set to default (ISO becomes auto for example) and WB seems be auto, even though it shows "daylight" selected.
This makes me worry a bit about the possibility of corrupting flash params, although it seems to return to normal on reboot. I haven't tried changing settings that are still changeable.
Moved wifi specific discussion from https://chdk.setepontos.com/index.php?topic=14294.0