1. The shear pins are not actually pins but more of the injection point for the plastic material injected in a groove around the inner shaft. I used to think they were actually pins also but recently Julesdawiperman posted excellent pictures of a disasembled shaft and they are not what they appear to be.

2. They should not slide easily if at all but if they do it isn't a problem as long as you are using the correct shaft for the correct steering box. If you are there is no way the two will seperate under normal operating conditions. If you are using the wrong shaft or swap from a power to a manual box and stretch the shaft out it may not stay together so to that.

3. If you are converting from Manual to Power do not collapse the column to make it shorter unless you properly modify the lengths to be the correct length for a power box shaft. Pushing it together will work but you will loose or reduce the collapsing effect and that would be bad in an accident.