1. you can not use your old brakes with the new wheels. the canti pivots are in the wrong spot. sounds like you already found that out though.
2. most mountain bikes can fit 700cc wheels on them. You might not be able to fit large tires (wider than 30cc or so) but each frame is different with different clearances. I.E., you have to check it out for yourself but you will most likely not fit regular size MTB tires.
3. whether or not your wheels are disc compatible should be easy to tell. Does the hub have a place, on the non drive side, for a disc to be bolted to it?
4. Why did you get 700cc wheels for your 26'' mountain bike?
I am not trying to criticize you. I am in the process of trying to build a 700cc wheelset for my Surly 1x1. But, I am sure it has plenty of clearance for 700cc wheels and I know the hubs will be disc hubs.