Merc Handbook stuff.
I think you may have focused too much on certain parts of the Handbook. It is very specific as to the nature of warfare for Mercenaries, but not house units. It specifically states that in major conflicts employers do their best to retain command rights due to Mercs acting like warfare is a game and not actually fighting it out.
As for what I suspect warfare is like in the early 20s, it's probably mostly raids. This was the end portion of the third succession war, and the FedSuns and Lyrans were building up to their massive offensive in 3025. Combat was low intensity and much of the industry was degraded. In the late portion of this decade the fourth succession war took center stage, with massive levels of conflict along all borders, and especially along the Terran corridor.
40's: This was immediately post the war of 3039, and most nations were in a rebuilding phase (particularly the FedComs and Combine). Combat was again likely mostly limited to raids, planetary assaults would have been pretty rare.
50's: The early portion saw massive combat between the DC/FRR/FC and the clans, but after Tukayyid this decreased primarily to raids. Towards the end of the decade the FWL and CC attack the central portion of the FC, creating the Chaos March (a virtual mercenary's paradise - pretty much any kind of conflict could happen here between mercs and upstart governments). A year or two later the Inner Sphere takes it to the clans in Operations Serpent and Bulldog, eventually virtually annihilating the Jaguars. Overall, most conflict during this period revolves around the clan borders with short heavier spurts appearing in the Terran corridor and around the CC.
