Welcome to the Forum!!
Keiffer trailers were always known to be excellent quality, heavy trailers that took a beating and still kept going and being reliable safe transport years ago.
Sounds to me your trailer needs some minor adjustments made to it so things close that should close and stay open that should stay open.
Your trailer is a 2001 ...18, near 19 years old and you must expect some things to rattle, make noise, not fasten as tightly as original and new..
I would contact the manufacturer and see if they had any issues with options your trailer has in it.
With a VIN number they know what it has, has not and if there were any recalls on products that happens periodically.
Sometimes rubber stops and hold-backs dry-rot or just loosen with age...
Latches can shift or bend slightly over time which could account for partitions to open.
As for horses fitting in slant positions...hay nets do take up space so that could be part of your problem.
Measure each stall and make sure the horse is placed in as size appropriate as possible a location.
I don't feel a "must-do" need to reward my horse for loading in the trailer...he goes where he is told...a pat and good boy is often all mine get.
I have a 2 horse straight load and a 4 horse semi-stock so don't have issues of slant dividers and not fitting happening often.
Try removing the hay net and see if it remedies your situation.
Your horse can eat outside the trailer if he needs while tied to it so he is occupied at your destination.
I would contact the manufacturer first.
Ask about recalls, or how to realign, tighten things that have loosened over time and see what they can suggest to help you.