Thats a lot of questions and you could write a small book to answer it. The military hasn't changed much maintaining custody of equipment since the CW. They had paperwork then and still do. The company CO was responsible for his men and their issued items. Inspections were performed and deficiencies listed. If you couldn't explain the loss of an item according to the regulations, you may have to pay for it.

Soldiers kept their weapons at all times unless on leave or in the hospital. If a soldier was transferred to another unit, remember his unit CO had responsibility for that weapon. As a rule he would have turned it in and drawn another in the new regiment.

As for making personal markings on weapons, it was done and it is was not uncommon. See this web site: http://www.angelfire.com/ma4/j_mayo/scabbard.html