wow been quite a complex conversation here !
2 ideas as my two cents to this topic :
1 - what about defeated lords staying longer in-walls training troops, so that they don't rush to war with an army of peasant-like meat balls. on the other side, the player would also be able to train their troops in town and castle, for a price if they're not owners.
'cos yes i feel sorry when i meet an enneny with more than half his troops beeing recruits... but well, ifeel happy for my own troops who will have some safe morning exercise.
2 - what about a radius (visible with the alt key) depending on your tactics skill, that would define wich allies and wich ennemy will join the battle (ok a bit like now) but instead of joining right at the start with this magical shift, each party will have a timer depending on their distance and map speed s that they'll join the next reinforcement wave only when their timer is elapsed.
this whould have two advantage : make the battle more dynamic giving you a chance maybe to beat your first opponent before a massive wave of reinforcement and leave with youru loot and prisonner, and on the other side, when odds are against you, you might aslo have to survive long enough and hold until your own reinforcements arrive.
and this has the advantage to still stop time on the main map (i guess because the engine can't cope with map logic together with the player battlefield logic) but gives the illusion that time still runs.
so just like player reinforcing an ally and change the tide of the battle, player-started battles will have the same level of dynamism.
(and what if the allied lord is really not your friend and might enjoy seeing you beaten ? but that's another step ...)