I've never tried that combination but I think it would have been likely to pinch flat with the too-large diameter tube, but I would have probably given it a try as a temporary fix, and ridden slowly. The presta valve would have been a non-issue. The fact that the road tube was not designed for as much volume as the mtb tire would have been much less than ideal also, but still might have worked to get you home..............In a sort of oppposite situation, I was mountain biking about a year ago with a guy who was riding a 29" (700c rim with fat knobbies) mtb; he had a flat, no tube, and he successfully stretched one of my 26" x 2" tubes over his rim and we rode on-
Yes you could for a short-distance emergency. I have done so on the bike of another person who had not brought an MTB tube with her. I used a 700x25. The problem is that the tube will keep the tyre reasonably inflated, but move inside the casing as the bike moves along (at least, that was what was demonstrated in this example). The other factor would be valve-hole size -- if the MTB ran presta tubes and you tried replacement with schraeder valves (of any size), it wouldn't work because of the differences in valve thickness.