Lots of issues in one post here.
Firstly, and a huge bugbear of mine, is loaners/sharers who say ‘I don’t want to pay to school someone else’s horse.’ The fact is that all horses (even ones with only front shoes on and not eating hard feed) cost a lot of money to keep, and all horses (even an Olympic gold medal winner) need ongoing schooling to keep them ticking along at the level they are at, and need even more schooling if you want to improve the level they are at. You can be sure a decent owner is also paying (and probably a lot more than you!) to school their horse.
Secondly, there is nothing more jarring to the ear of a horse owner who is actually quite happy with their horse than the sound of someone not being happy with their horse. They might think their horse is perfect in every way. It might be a complete knob or just evil, but if they like it and are happy, a loaner pointing out its faults is not going to endear themselves. It is also possible that a loaner’s inexperience has allowed some more unsavoury behaviour to come to the surface. I have wonderful helpers for my horses, but I did have to spend a lot of time and get in a remedial trainer to help me re-educate my horse after one rider allowed a mounting problem to get out of hand.
However, having said that, there are some decidedly naughty ponies out there with poor manners and unpleasant personalities, even though their owners love them. You have to make a decision as to whether you carry on with the pony or look for another one. It honestly doesn’t sound much like the sort of set up I would be much won over by. If the pony, owner, yard and hacking are not doing it for you, there are lots out there that might be a better match. One of the huge advantages of being a loaner is you can just walk away from any situation you don’t want to continue in. I wouldn’t tolerate nippy, nappy, or bad tempered in any way. WRT not being great in canter, yes that probably is a bit of your fault. Spooking - sometimes this is the horse, sometimes the rider, usually a combination of each of them not trusting the other. My DD’s horse is perfectly fine when being ridden by DD, but a recent new sharer has had nothing but trouble with him. Spooking, napping, refusing to be mounted, etc. We’ve owned the horse for nearly 4 years and my daughter has been riding him since she was 12, including competing at a vey high level (British team in her sport.) So we are pretty sure it is not the horse that is the problem. If it’s not the horse, we can only assume it is the rider! But the rider is convinced it is the horse.
I can’t advise you what exactly to do, but it is worth looking at your relationship with the horse and owner and deciding if it is not working for you. If it’s not, go and look for something that works better. It will be better for owner and pony too in the long run.