So, a few things:
On the belt, I haven't had issues with slipping, BUT I did have a 501st member help me dress the other day, and he threaded the belt wrong. Like Kristen said, sometimes folks just don't know how it's supposed to go, so they guess.
On the holster, well *eye roll*. So dumb. On so many levels. Kind of like making a brand new belt while in transit on the Falcon. But it is what it is, I guess.
And something new that I discovered. Might not be new knowledge, but I'm not on Facebook, either, so...anyway...a friend sent me screencaps of a tutorial from FB about how to make the seams in the trousers. I did so with a practice pair and messed them up, so then I did another pair, and tried something else. I didn't cut them. The tutorials I've seen have you cutting the knees and then sewing them back together to make the seams, but I just put the pants on inside out, drew the chalk lines I wanted, took them off and neatened the lines up, then sewed the seams in. That way, if you mess up, you just pick them out and redo them. No muss, no fuss. Super easy and took all of half an hour.