This looks good.
Bravo.

1.) Having all data for each merc in individual files would be a very cool way to go.

2.) But the problems I can see here are that either ALL data has to be externalised at once or we are going to have a series of incompatable .xmls (giant ones at that) everytime something new is externalised.

3.) The chances that the coders will drop off the edge of the Earth in the meantime (while we wait for fully externalised features) are astronomical.

4.) But you are right that the most convienent and certainly most mod interchangeable method would be for each merc to have all data contained in one file.