I would really like to go to bed, but the slow cooker isn't done I've got a 6.5qt slow cooker going, with about 7lbs of corned beef in it.  I had put it on HIGH around 4:30pm, figuring that it would take six to eight hours to cook.  Well, it's half-past midnight, and it's not done: the meat is chewy in exactly the way corned beef shouldn't be, due to an abundance of insufficiently cooked connective tissue.

What do I do now?

If I put it on LOW and go to sleep, it will be eight more hours of cooking, which seems like rather a lot.

I'd been planning on cooking the cabbage in the broth, after the meat was done, overnight.  I could fish the meat out, put it in the fridge, and go ahead and cook the cabbage, then tomorrow morning, swap out the cabbage and continue cooking the meat.  Since I have a commitment tomorrow, that means I'm still going to be stuck leaving the meat in the cooking for an additional 5hrs minimum.

Should I leave the meat in there on WARM overnight?  Will that (1) be safe, and (2) be sufficient to cook it?

No, I don't have a slow cooker with a timer; no, I am not waking up in the middle of the night; no, I can't take the ceramic liner out and park it in the fridge (it's bigger than my fridge.)
Warm isn't guaranteed to be safe. On some cookers it might be, but unless you've checked yours already, you can't say for sure. I suppose you could try it and check the temperature in the morning, and if it's above 140F everywhere, it's okay, but I'm not sure you want to risk that.

So I think you're stuck either cooking it overnight (on low, not warm) or moving it to the fridge and cooking more tomorrow.