Lots of questions and I'm not quite sure by what you mean by some of them.
You can spread the frame to 126 mm assuming we are talking about a steel frame:
Bicycle Frame/Hub Spacing
You can simply push apart the drop outs to mount the wheel; it will be a little tougher getting the wheel in and out though.
Not so easy to change the dimension of the wheel from 126 mm to 120. You can put in the correct length axle and spacers but you will also need to redish the wheel since the extra length will come out of the freewheel side. I doubt there is much that can come out of the non-drive side in terms of spacers and the like.
A quick release can hold the wheel in your dropout; that is what it is designed to do (this has nothing to do btw with whether you are running a clincher tire or not). Do you know how to properly use a quick release? If not, google around and watch a youtube video.