Thanks for all your comments and your help :-)
#4 - Yes, I used a flat file and I think I left a bit of a sharp corner.
It's clear to me now that I did a poor job of enlarging the dropouts, but given the number of people who attempt this “workaround” with electric kits, I think biking forums (at least others I’ve read) should stress the importance of installing a torque arm to compensate for the powerful torque delivered by these motors. Simply filing away leaves a problem waiting to happen (especially when a strong force is applied to the front lip, which is usually thinner and more likely to snap).
As for me, it seems clear that the next fork will have to be new (no repairs or part swapping) and have 10mm dropouts (although most models I’m seeing are 9mm).
Live and learn (if you’re lucky :-D).
Cheers,
Alex