I am American and I say both could care less and couldn't care less. I never understood the could, because you're right, it doesn't make any sense that way. Someone once suggested to me that perhaps if you say I could care less you're being sarcastic, but I don't think so. My theory is that it comes from I couldn't care less, which got turned into a set phrase, so people stopped breaking it down to understand the meaning, and as long as no one is looking at the literal meaning, could and couldn't actually sound very similar and it could easily get changed from one to the other.

I'd never thought before about saying 5 of 6 for time. Don't know what the of is doing there, but I would probably personally say 5 till 6. What really confused me was when I first heard British people saying half eleven, and I couldn't decide if that was going to mean 10:30 or 11:30. I think someone told me that in German half eleven would mean 10:30, or maybe it's Russian...But in British it means 11:30