I get that you're new to cycling Moisture, but some of your questions and comments are rather comical. Why the hell wouldn't you be able to just ride the Knobby tires to the trailhead on pavement, then ride the trail? If you're planning on switching wheelsets, you would need to bring both wheels... front traction on singletrack trails is at least as important as rear traction.

Change out the fork for rigid? Then you are defeating the only thing that gives you a (albeit limited with that old fork) advantage on single track. I agree with the previous poster's comments.. Also, it certainly looks like your bike does not fit you, but you are forcing the issue with add-on extension and an upright stem. I would suggest reading more about cycling, both road and MTB. There is a wealth of knowledge here.