Thank you again, chvid! Yes, that is a Giant Revive. Hubby and I had searched but hadn't found any torque arms that would work on forks such as those. That torque arm might well work!
The motor is only 200W (maximum allowed size to remain classified as a bicycle over here) and I had done over 200km on fairly rough terrain without checking the wheelnuts, so I admit to total stupidity.
I'll remember to check all nuts regularly from now on, but I'll see if I can get a pair of those arms for peace of mind.