Thanks to everyone here for trying to find ways to make this horribly broken game playable. Haven't posted previously, but I've been lurking since the original thread was posted in January.
@crinrict: I was able to replicate your success in using overwatch to salvage a game saved just prior to the invisibility issue occurring.
For background, I was playing the base game, WA, Ambitions, and LN on a 4 year-old laptop with performance settings adjusted to account for hardware limitations. No custom content other than a few store items. The game was patched to 1.18. Story progression on for the most part, along with normal aging. Admittedly, I hadn't played TS3 extensively prior to installing LN, but had never experienced significant problems with the game. After installing LN, I noticed a lot of lag, had issues with maids and butlers not showing up, extra cars in inventory, etc., and found it particularly difficult to play Bridgeport and Sunset Valley. For whatever reason, Twinbrook seemed to be fine, so I set up house there.
All was well with my family up through the middle of the 27th(!) sim week. Then it happened. The kids wouldn't stay in school. The dad (business career) wouldn't stay at work. If I sent someone off the lot, they got stuck under the stairs on their return. Sims started going invisible. It didn't seem to be linked to any particular event in the house. There was a pregnant sim, but she was 2 days away from giving birth. Two others were about a day away from aging up, but the aging hadn't been triggered yet. In short, it was just another ordinary day. I exited the game without saving, backed up the file, and started looking for a fix.
I tried evicting the family from the house. Didn't work. Tried moving them to Riverview—painful, because it meant losing a carefully constructed, very circular family tree :) They were fine for about 3 weeks in the new town, but then invisibility returned. Tried sending them to China. All was good on the trip, but the game screwed up as soon as the family got home. Tried resetting the game files. No success there, either. I figured it would be pointless to reinstall the game, so I didn't bother. I gave up on recovering my family and tried a starting few new games, but the inevitable lagging, freezing, and fear of invisibility prevented me from really getting much enjoyment out of the game. At that point, I basically quit playing.
I recently bought a new computer and reinstalled the game. This time, I didn't add WA to the mix, because I wasn't really playing it. So now I've got the base game, Ambitions, and LN. The game is fully patched, and I decided to install overwatch. Lo and behold, I was able resurrect my previously unplayable family. No lagging, no freezing, no invisibility. I've been playing them for five sim days, and so far, the game is functioning well. Now to see how it holds up over time...
Note: I did not test the borked save file on the new build prior to installing overwatch, but I'm pretty confident the sims would have gone invisible right on schedule without it (a theory supported by crinrict's test of CoffeeChick's game).
