The scraping of the coating you report tells me that it's both. The hollow base of the Hornady swaged WC's is designed to expand under pressure to grip the rifling, so the fact it's a little under diameter makes no difference. Your plated bullets are sized like jacketed bullets because the copper is tough enough to resist gas cutting if the fit isn't dead perfect. The coated bullets, like hard cast bullets are sized a thousandth over to ensure groove fill to avoid gas cutting, which leads a barrel and compromises accuracy.
So, what should you do? There are some options. If your cases are squeaky clean and polished smooth on the inside, it will exaggerate their friction with the coated bullets. Cases don't need to be shiny, though I find it does make them easier to spot on the ground. One thing you can do to avoid the friction shine is tumble with Nu-Finish or some other car cleaner that leaves a little coating on the surface. If you pin tumble, using a self-waxing car washing concentrate instead of dishwashing liquid will have the same effect. These coatings may provide enough lubrication to ease bullet seating, but you'll have to experiment to be sure. Using a dry lube on the bullets would be next on my list to try. That is, put them in a plastic tub with some motor mica powder or graphite and shake them around until you get a coating. Alternately, you could stand the bullets up on a cookie sheet and apply a spray case lube, like One Shot, and let it dry for a day. An alcohol solution with Lee case lube could be sprayed on and allowed to dry. You could put graphite powder into a pump sprayer with alcohol to suspend it and spray that. Just mind the flammable fumes. How much graphite to add, you will need to experiment with. The Lee lube, which leave a white dry lubricating layer behind, is usually dissolved 10:1 in alcohol for a spray case lube. You might get away with thinning it even more in this instance.
