...some case mouths barely crimped at all, while others rolled clear up under the crimping ledge in the die and distorted when I forced the ram through its stroke.
I'd take a serious look at your seating die if I were you. Straight wall cases that are a couple of thousand shorter or longer just don't do that. I've been reloading for 40 years without trimming handgun cases and never have seen anything like you just described.
Don