So I entered the deep dwarf mines using the mine cart and the password and I foolishly saved inside the deep mines!
Can I leave early or do I need to kill the dwarf king on level omega in order to leave?
thanks!

You can leave at any time you want, it´s an open RPG. A word of advice thou, keep multiple saves. You never know when you might be in a hot spot;)
Enjoy the games!

Good to now thanks, how do I leave exactly? The mine cart wont bring me back to the surface.

You have to tell (write) what level you like to go to when the mine cart asks you. To go to the entrance tunnel type Mine 1.
Here is a good site if you get stuck: http://shrines.rpgclassics.com/pc/mm4_5/
If you can�t find your answer there just ask here on this board:)

I'd like to second that nomination. RPGClassics is a phenomenal site. Here's another Xeen site I use that tends to fill in whatever blanks RPGC doesn't hit...
http://www.gamefaqs.com/pc/564553-might-and-magic-v-darkside-of-xeen/faqs/44322
...the guy who wrote it, Andrew Schultz, was a major M&M player!