I do not know the answer to your first question but I know that because ribs and such can move that just because you can put your fingers behind it doesn't necessary mean that it couldn't be pinching. Usually compression is not an all the time thing, and rather in certain positions, compression occurs and it causes damage over time.
As for the second question, if you press on an area and it is tender it does not mean that's where the compression is. There are many different reasons for pain. For example, my pec minors are very tight and hurt to press on BUT the compression was seen between my clavicle and first rib. The compression there causes other muscles to spasm which causes referred pain. Nerve pain is difficult because when you apply compression to any part of a pissy nerve it can radiate under the same principles as double or multiple crush syndrome. Thusly, trigger points.
Anyway, hope that helped at least a little?