Hi there Dix:
I'm no expert on dealing with these situations but I just want to offer you a few of my thoughts.
What you are going through is so very difficult and frustrating. I'm so sorry that your boss is behaving so Nish. I've worked with people like this before and I know my first instinct is throttle them (which is totally against my every day belief system and shows the extent to which they are capable of making me...crazy!

).
I deal with those thoughts by consciously reminding myself that I don't believe in throttling people and that throttling is wrong and that I am not a throttler.
In your case...it seems you are feeling overwhelmed and unable to cope. The doc/T haven't helped enough by extending your leave and so now you are faced with going back to work with this person....which I sense you are very much not wanting to do.
I think there are a couple of things you can do:
1. Call in and explain that you need some of your holiday time immediately, due to personal circumstances (not sure if this would cause you to be fired....or if you might be allowed to do this. Is there someone above your boss that you could make this request to?? Without explaining...other than saying it's an urgent personal matter?
2. Quit the job.
3. Decide to make some changes to your thinking....to help yourself cope and feel not so overwhelmed.
1 and 2 would end or temporarily deflect your problem. So then there's number 3.
You could decide to play a game with this N. If you were to think of your interactions as more of a game, and picture yourself winning, no matter what it takes.....you might feel better about the whole thing.
It sounds like you feel as if you are showing weakness or being weak...because you are having a tough time dealing with this bimbo. If you were to decide...that this person has no power over you, and that you are not going to allow this dope to gain any power over you, you might feel stronger.
If you were to brush off, as much as possible, the things this doughhead says and does, and consider his/her words and actions as that of a spoiled brat....without a head.....and picture the person in some ridiculous form....having a big happy face painted on his/her forehead....or wearing the underwear of the opposite sex....or something equally silly.....you might not take everything this person says/does so seriously......and thereby...reduce the power he/she has over your emotions.
I know this is not easy to do, especially after a few years of being batted around by this dimswitch, but it is possible to go in there.....with a totally new attitude....smile and be pleasant......ignor as much idiocy as humanly possible.....respond as if to a wee child, who knows very little, .....smile some more.....walk away and do your job.
The idocity of this moron.....to phone you at home when you are ill and ask you to deal with a work-related problem......shows how totally and utterly un-empathetic, self-centred, power-hungry etc etc etc this person is.
The choice you have is to say......so what.....he/she's a moron.....I'm not going to let him/her p in my cornflakes.....as a matter of fact....I'm going to eat my cornflakes before I come to work and bring strawberries for his/her cornflakes. It will totally freak him/her out!! The fact that he/she is no longer able to p you off.
The more I think about it.....he/she is probably jealous of you!! Of your capabilities/assets/qualities/relationships/life etc. Smile and keep on smiling, is my best advice, if you must return to work. Give no indication of reaction to silliness/nonsense. Just smile and do your work. Agree with anything slightly reasonable. Notice whatever good things you can and point them out. Refuse to let this person belittle you....just smile and walk away...of if that is not possible....respond kindly/politely/genuinely unaffected by trivial attack.
Best of luck to you Dix. I wish I could think of some better choice. Maybe others will?
GFN