It's been complicated by the fact that the crashes I was getting seemed a bit random, so instead of just loading a particular house or lot, seeing if it worked and then adding back more of my CC, I had to load several lots. My process was this (I crashed while doing all of these things at least once before I started testing):
1. Load Carmody-Moretti house.
2. Send Kimberly to Town Square and back again. Back out to the hood screen.
3. Load Nott House.
4. Send Lia to the library in her car and back again. Back out to the hood screen.
5. Load Moretti 2 house.
6. Send Aurora to my new teen hangout lot and then to Sullivan Mall and then back home again. Back out to the hood screen.
And that was each and every time I loaded the game. I've probably loaded the game somewhere around 40 times over the last few days. I'm pretty over it!
The folders that I crashed with were my Plate Replacements, Walls and Floors and my Sweet Down Low painting replacements. If I'm correct and I've solved the issue, the problem with the last two folders was that I had duplicates of some of the files in other folders (this is a fantastic, easy-to-use program if you want to check for dupes in your own game). I also redownloaded the Sweet Down low paintings just in case. With the plate replacement, I don't know. I guess it just corrupted on its own? How fun that files can do that for no apparent reason, right?!
Anyway, I'm too cautious to say I've fixed the problem but I did just manage to go through that whole checklist with all my downloads in (minus the plate), without any crashes or glitches. Tomorrow, I'm going to try to play for real. I have a couple of other things I'd like to try if this doesn't work, but they're all quite annoying. So keep your fingers crossed for me!