1. It all depends on what machine you use to mill the datum, so I'm fortunate enough to have a VMC at hand. (when i use this method i get a parallelism of 2um)

2. That would be true if you could exert a serious amount of side force to dislodge the rails, but in this instances its highly unlikely to happen.

3. Its tried and tested and works fine; all depends on the application and I think it would be necessary for the large machines that are doing heavy milling or moving heavy loads.