Well.. This is very frustrating and strange to say the least. I tried deleting and importing via a .csv through the UI. Errors. I tried deleting and importing via CLI. Errors. Then I went to double-check the filesize. Ok.. That's interesting. The first twelve files were ~14MB while the rest were only ~4MB. Looking into it deeper, it looks like those twelve were actually .tif files that were somehow named as .jpg. I rescanned the slides, made sure they were in fact .jpg of the correct size, and reimported them. No more errors.
Very odd that they had the wrong extension. Probably some sort of glitch with the scanner software during the initial scan. Yay, more troubleshooting, but at least it seems to be an isolated incident.
Thanks for the help yet again!