Sorry if I've missed something, but is the problem just that the bolt won't turn to loosen?
Have you tried a good quality 6 point socket, extension and a breaker bar? Extension is to give you enough space to use the breaker bar. That should be all you need in this situation.
If the nut is rounding off, get a set of bolt extractor sockets and hammer one on, it'll bite into the nut.
Also side note, it's not a weird place for a locking nut, the engine vibrates. Vibrations loosen nuts and bolts. It's perhaps not a NECESSITY, but it's actually quite common practice.
Have you tried a good quality 6 point socket, extension and a breaker bar? Extension is to give you enough space to use the breaker bar. That should be all you need in this situation.
If the nut is rounding off, get a set of bolt extractor sockets and hammer one on, it'll bite into the nut.
Also side note, it's not a weird place for a locking nut, the engine vibrates. Vibrations loosen nuts and bolts. It's perhaps not a NECESSITY, but it's actually quite common practice.