Well here's the thing you all need to understand. As a supervisor, we have jobs the same as in which we get yelled at more than you. If you're not getting done on time (especially consistently) and misloading frequently, it is a supervisors job to fix those things. We are not out to screw you, because that just screws us. We don't want to be crude with people, but with the culture that is UPS, it is most of the time necessary. And let me just say, a talk with a supervisor, part-time or full-time, IS NOT HARASSMENT, it is our job. Unless they came at you cussing, yelling, or insulting then they are not harassing you. We are not a production driven company, but unless your methods are great, they have every reason to talk to you and attempt to fix the problem.
Based on my experience, the stories of hourlies and sups are neither true exactly and is really somewhere in the middle. I don't know these sups or how they have been treating you exactly, but I expect you are blowing it out of proportion. They should be working with you to make you as best as you can be, because once you know the methods, you can load more without working as hard.
As far as help at the end of the day, you cannot refuse it. Your work area is considered you area of the building and not necessarily your three or four trucks. A supervisor is responsible for that area getting done, and they may use whatever people in that area or outside that area that are willing to do so.
If they are calling out the misloads at the beginning of the day, I guarantee you they are not making them up just to be dicks. The more misloads a sup has on their area, the more we get yelled at. The misloads that are reported by the driver show the truck the package was assigned to and the truck it was discovered on. If you consistently misload, which we have records of, then that is a problem outside of "my help did it!" The only way to clear the air on that is to run a check on the package to see who scanned it. However, misloads are not always scanned so this is as good as considered yours unless your scan percentage is 100% and your help wasn't scanning at all. As far as having them pull your packages while you load, that depends on the speed of the loader. If they are faster at loading, it only makes sense to have them load the trucks for you at the end of the day.
I know how it feels as a package handler to be damned either way, but I promise we are not trying to screw you. I don't know your sups, but I'm at least 75% confident they really aren't trying. Catch one of them after the shift and talk directly to them without threatening grievances and ask for tips, I for one would love to help out a loader that wants to get better. I won't be as willing to help a loader that is making excuses to not get done.