Update (3 hours later): Not happening. I
will go ahead with the csv import to get the rest of my books on GR tomorrow or the next day, but I don't have the patience for syncing. It's not the plugin, it does what it's supposed to. It's just that the link between GR and my Library isn't vital enough for me to really want this bad enough to do the work involved. I know, I said it would be "heavenly" but I think I was just feeling ambitious, because after getting only a handful of books linked and shelves imported, I'm just not feeling it.
What I may do instead is use my GR Shelves column to list the shelves I want the books on, because that will export with the CSV and into GR if I modify the column title. Kind of like syncing pre-emptively (and manually). If there's one thing I know how to do, it's make GR-compatible CSV files, so I'll stick with my strengths.
Obviously, I'm not going to go back and do all the ones already over there, but this will make close to half of them match.