Nice. Looks simple and flexible.
You're absolutely right I think about the destruction of armies. In a single battle (I don't play SAGA but most games have the mechanic I guess!) a model being 'dead' means 'unable to take any further part in the battle' - which may only mean unconscious for half an hour, suffering a bad sprain, lost their equipment, temporarily lost having run away, stuck in some mud or whatever. Incapacitated during one afternoon doesn't mean you can't be fighting fit the next morning, or next week.
If there are different battle scenarios in SAGA (you mention 'the battle at the ford') can you tie others to different terrain types/locations (eg, if there's a 'loot the village' scenario, that's what you do attacking villages, if there's an 'ambush in the pass' that's what you use in the hills, etc)?
You could also determine 'initiative' somehow - I don't know what initiative system SAGA uses, but it would probably best be a meta-game thing anyway, something like 'each commander rolls a d6 and adds the number of points of their army (larger army = more scouts = greater knowledge of enemy movements), and the highest gets to chose whether to be attacker of defender'. This means you could have a situation where the vikings (attackers) try to loot a village defended by Normans (defenders), or where Normans (attackers) surprise a viking raiding party that has attacked a village (and they're now defending it).