On Experience: I know that at least up to MTW experience is recorded for every single soldier (if you´ve got logs turned on, you can read battle logs which show what happened to each and every single soldier in a battle), but what´s displayed ingame is an average. So whenever you retrain a unit in MTW, its average experience gets diluted, but it keeps its high-experience members. Actually, it can happen that even a few unlucky losses dramatically lower the unit´s experience, if its veterans are killed.
I don´t know whether this still is the case in RTW and M2TW, though.