Re: which is
I wouldn't. You may hear something like this:
... in Modesto, which is [pause, when you realize you don't know] I don't know where.
But you wouldn't say it as a continuous utterance.
1-He says he is a cardiologist, but I don't know what that is
2-He says he is going to meet the famous Jane Hopkins, whoever that is.
3-He says he is in Lory, California, but I have no idea where that is.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.