Update, manual defueling is reported to ruin the solenoid valve seating and require replacement.
I think these folk are talking about the same style of valve:
https://www.cngchat.com/forum/genera...bad-tank-valve
They talk about 12v opening a pilot valve which operates the main valve. I'll sit on it, wait for warmer weather and research tank replacement options. At least now I have the bolts for the cover worked loose and can take accurate measurements.
Screwing off the shut-off valves (~20mm hex with 4.5mm allen key in center) and applying heat and penetrating oil is of course a job for a qualified cng technician.