@JayMartin I have bent my rear mech arm several times. Each time I straightened it the best I could and then put it back on the bike. Stand over the mech and cycle through the gears, look for the jockey wheel alignment with each other and the cassette. If they are out of alignment, it is more important that the upper jockey wheel is in alignment with the cassette. If necessary, have another go at straightening the mech arm. It may just require twisting. Be aware where the twisting force is going, so hold the top of the mech arm and twist the bottom bit, use tools if you have to do so. Make sure that the two plates that form the mech arm are open enough where the chain enters to allow it to do so without rubbing.
You may end up with a mech that is pretty damn good but is out of alignment with the bike and causing shifting problems. In which case you may need to bend the mech hanger. For that you really need a mech hanger alignment tool. They can be expensive (£10-£85)*, but the job is very quick to do, so bike shops don't charge much for hanger alignment. The last LBS charged me £10 for mech hanger alignment, but that was about three years ago.
I have completely snapped the original XT mech right through the clutch body, so I had to replace it with whatever mech I could get, an SLX one. Yesterday I discovered that one of the teeth on the lower jockey wheel had snapped off. I thought I would replace the broken SLX jockey wheel with one of the XT jockey wheels. Don't bother, it doesn't fit! When I tightened the axle screw, the wheel would not rotate. Back off enough to allow rotation and the screw is too loose to stop in. (By the way, the XT jockey wheel has a proper ball bearing, whereas the SLX is a bush).
I refitted the broken jockey wheel and it still worked perfectly on today's wet and gritty ride.
I now have a spare SLX M7100 mech, just in case.
*later edit
