You bring up some good points, and certain types of magic could be challenging to integrate if one is concerned about verisimilitude, but I think I disagree on some points (numbered so they are easier to discuss).
1. Units are made up of multiple stands working together. When a unit loses half its stands, it has a chance of morale failure. The advantage that area of effect attacks give is that they can be positioned to hit multiple stands.
2. While a solo should be able to do something besides just attack, the granularity is not their. This is not D&D scaled up. It is a completely different combat system designed to scale up to a wargaming system. The time scale is different; the action economy is different; the movement is different. In normal D&D, a creature gets 10 actions per round. In Battlesystem D&D they get 1.
3. Some spells get wonky. Disintegrate and Finger of death could kill a stand even though I think it usually only works against 1 creature. This is a substantial bonus. Area of effect spells can do damage to multiple stacks. I think this offsets the wizard only getting 1 casting per round.
4. Combat is an abstraction in D&D. Wargaming combat with groups or creatures as stands or units is even more of an abstration. Don't expect it to make sense very much. Expect it to be as simple and fair as possible. (And yes, I consider allowing spells that target 1 creature to affect an entire stand without burning more spell slots to be a pretty good trade off. Without the abstraction, a fireball could conceivably miss all the creatures in a stand if they were at the outside corners of the 20'x20' square.
5. The solos probably should not get extra actions as it throws off the normal progression of the combat. Solos will already have actions, bonus actions, reactions the same as stands. Each of those stands is only getting the normal set actions, bonus actions and reactions as well, even though they have to fill a minute as well. A stand has multiple units in it. In normal D&D, some creatures could take actions while others attacked, healed or did something else. Stands honestly have a stronger argument for multiple actions than solos. But this is Battlesystem D&D. Everyone only gets the standard set of actions per round, even though it is a minute.
6. The fighter will already be able to multiattack the one stand or multiple stands with its movement in squares in between attacks. It gets its full set of actions for the round. Likewise, a rogue could use an item as a bonus action.
7. The thing to remember is that all the creatures are doing other stuff, this is just how we are abstracting it.
In short, I think that Battlesystem combat will work best if it is treated like normal D&D combat with the minimum of changes. Something similar to the following might work.
1. 1" = 1 square or 20'
2. Stands/Solos get their movement divided by 5 to get squares per round (or squares * 20 feet. So 30 feet of movement changes to 120 feet of movement per round. So multiply movement by 4 to get feet per round.
3. Stands that attack solos that are on their own make as many attacks as creatures in the stand.
4. 1 round = 1 minute.
5. Moral failure rule.
6. Solos can move with a stand and act independently but get protection from being mobbed.
I probably am missing something, but I could see the players at my table being thrown into a situation where they have to lead an army and being able to grasp the rules quickly because the action economy is not messed with.
