The description on the invoice sounds very reasonable. Getting to it may not have been too difficult, but troubleshooting the circuit to determine that was the issue certainly could take some time. It would be worth checking. As to were the ignition switch is located on the 1991 Escort, I won't be of much help there. Keep in mind that the "ignition switch" is not the same as the "key cylinder switch." The ignition switch is likely mounted in the steering column and can be probably be accessed by removing the shrouds covering the steering column. You might have better luck asking for the location of the ignition switch on the Escort-specific forum.
As for what may cause the screw to come out initially, it may have not been tightened properly following a prior repair, or maybe it was cross-threaded at one time and the threads no longer engage the screw securely. If the screw were not able to tighten up though, I would suspect that the invoice would have documented that as well.
-Rod