c) use an earthing nut instead of the locknut and tighten the gland down into it as they have teeth to grip the enclosure.
d) use an adaptable box or conduit box to gland the cable into.
e) you could probably do something creative with a pair of Conlok adaptors and a short bit of conduit.
f) gland it to the side or bottom of the board, it sounds like the install is already a mess so an ugly cable bit of cable routing probably won't be too bad.
There are often many means of achieving the same ends and, as I've previously stated, taking a hammer and chisel to the lock nut would be a last resort.
Sometimes an earthing nut isn't available in the required size - unfortunately they aren't widely stocked by local wholesalers, even in small sizes.
Often glanding into something other than the DB is a viable option, but sometimes not.
As you have stated, gland and locknut form an electrical connection. While I've become increasingly confident about the use of Conlok, it should be remembered that the electrical connection depends on a 2mm grub screw.
Glanding to parts of the board (other than the gland plate) would be a usual enough option, but sometimes space constraints mean it isn't possible to get the cable in question to anywhere other than a busy gland plate.
For clarity, and repeating a point I've made several times already, I don't like taking a chisel to locknuts but on some occasions it can be the only option and is a viable (yet rough) method of securing a tight connection between gland and plate.