Reporting back, I didn't try reducing the screen resolution yet, but running the program outside of Steam (presumably to further free up system resources) didn't seem to help. I played for close to three hours, and experienced two crashes during that time. I don't remember how many crashes per hour I was experiencing before, but it doesn't matter: we're still crashing, seemingly at random.
Now there's a worse issue for me, though. After updating to OS X 10.8.2, I'm suffering from the "black screen syndrome" that some others had described months ago. Starting the game results in a black screen and no sound. I've tried changing the WINE engine and it didn't make a difference. It's possible that the timing of this problem with the update to OS X 10.8.2 is just a coincidence and that something else got screwed up before then...
I'll keep working at it because I enjoy the game, but I'll be avoiding any non-native games in the future.
Edit: I couldn't resolve the black screen issue, so I loaded Steam with the intent of trying to validate my files. Before I could validate, it began to download an update. Finder indicates that the version went from 1.6.0 to 1.6.1, and the Wine engine was changed. The update totalled a bit over 150 MB. Unfortunately, this didn't fix my issue. If anything, things became worse: instead of getting a black screen, I didn't get anything - I was just starting at my desktop as before. I tried validating through Steam and it would tell me that 11 files were problematic and would be re-downloaded, then 7. It seemed to find problems after each attempted launch. Finally, it said there were no problems, but the issues persisted.
I'm now reverting to a backup of the game that I made before I changed the Wine engine. If it works, great - I'll do the update through Steam, and pray that it fixes everything for good. Will report back again.
Edit: No changes, however I discovered
a post on the Witcher forums mentioning the patch, and also mentioning that OS X 10.8.2 causes major problems with Wine. So, while the patch is supposed to do some wonderful things, it doesn't do anything for the OS X 10.8.2 problem. I guess I'll have to wait for Wine and/or Apple to make some fixes before I can play at all.
Edit 9/22/2012: The problem with OS X 10.8.2 and Wine was patched. I upgraded Wineskin to 2.5.8, but couldn't get The Witcher 1.6.1 to work even with updates and engine changes. I restored my Witcher to version 1.6.0, then updated the wrapper from 2.5.7 to 2.5.8 and changed the engine. The game plays! It seemed more stable at first; I went for an hour and a half without a crash... but it was a crash. The 1.6.1 update to The Witcher uses a different Wine engine; whenever CD Projekt Red releases 1.6.2 with the updated Wine wrapper, I'll be eager to see if their new engine provides better stability.
Edit 10/18/2012: Version 1.6.2 was released some time last week, and I'm afraid to say that it's a major disappointment. To give a sense of how bad it is, I'll say that I played through chapter 2 with an unofficial Wine engine and wrapper, and only experienced a crash roughly every one to one and a half hours. The timing was such that the crashes became somewhat predictable. Chapter 3 brought about more frequent crashes, but the game was still playable. With version 1.6.2, it seems like there's a 50/50 chance of a crash whenever there's a scene change (which includes entering into a poker match). It seems like it worsens in the Trade Quarter, but the crashes are still frequent in the Temple Quarter that was originally so stable back in chapter 2.
To ensure that I was getting it as stable as possible, I deleted the entire game and re-downloaded it from Steam, and also made sure to do a fresh reboot before playing. I might have to go back to using one of the unofficial engines, but I'll admit that I'm not hopeful. I could deal with a crash every one to two hours, but worrying about a crash every time I try to enter or exit a house really ruins the experience. Saving every few seconds, and then having to reopen the game and reload the save, feels like a huge waste of time. I'd like to avoid setting up Boot Camp, but if the stability only gets worse from here, it may be the only choice.