Update- this appears to be an issue that varies by device/browser/OS. Best I can tell, even using essentially the same procedure, some are finding their content pastes fine and keeps format, others find the line breaks stripped and then have to add them again in the RP editor.
All of the product uploads are now being created in vBulletin 4.2 posts for consistency, and also to have backups and to log who creates them, so the source is the same now. Of course, for those additional people submitting reviews, I cannot standardize whether they use the RP editor to create content or copy/paste from reviews they have submitted on other websites or created on their computer or wherever
Anyway, it's much better now since proper formatting can be added, regardless of what happens if they copy/paste formatted text. Prior to the fix, extra spaces could not be removed so it's definitely an improvement. I think the only consistent observation from one person to the next is that the RP editor does not show what will be displayed on the page once the review is saved.
Thanks again for the solution! The website is finally looking nearly done and hopefully I can open it to visitors next month. It only took over a year lol.