Here's my attempt at revising the rules from http://media.wizards.com/2017/dnd/downloads/2017_UAMassCombat_MCUA_v1.pdf:
(1) Every mass combat turn takes 10 minutes, not 1 minute. (This is an aesthetic choice to make battles feel right; choose a different timeframe if you prefer.)
(2) Use everybody declares/everybody acts resolution, like BattleTech or AD&D, instead of turn-by-turn resolution. This is important for resolving battles.
(3) There is no Attack action, only a Fight action. When a unit Fights another unit, both of them are fighting and either one can take damage. See below.
(4) Resolve movement before resolving Fights. You don't need to Disengage unless you were already adjacent to the enemy at the beginning of your turn (during action declaration).
(5) When a fight occurs, you total up the BR of all allies involved in the Fight on each side, and roll [B]3d6 * (BR/100[/B], not rounded). The enemy units in the fight must lose that many BR--the enemy commander(s)/players can allocate the losses wherever they chose. Whoever loses the most BR is the loser and must make a morale check or disband and be destroyed. There is a cumulative -1 penalty to the morale check for every 5% casualties the unit has taken.
Example: If 200 BR of dwarves are Fighting 300 BR of Yetis while 150 BR of elven archers fires arrows at the Yetis, the dwarves and the elves roll 3d6 * 350 and the Yetis roll 3d6 * 300. If the elves and dwarves roll 11 and the Yetis roll a 12, then Yetis lose (11 * 3.5) = 38.5 BR, rounded down per usual 5E rules to 38. The elves and the dwarves lose 12 * 3 = 36 BR, which the dwarven commander allocates to the dwarves (because that makes sense, since the elves aren't in the melee and Yetis don't have spears). The DM is playing the Yeti commander and allocates all 38 BR to the Yetis. Since the Yetis took more BR damage, the elves and the dwarves win the field, and the Yetis must make a DC 10 morale check at -2 (they've taken 12% casualties) or be disbanded. The DM rules that the Yetis are normally Stalwart (+4), so the Yetis roll at +2 total. They roll a natural 14, for a total of 16, and remain intact. The Yetis and the dwarves will continue to fight next turn.
If I esteem mankind to be in error, shall I bear them down? No. I will lift them up, and in their own way too, if I cannot persuade them my way is better; and I will not seek to compel any man to believe as I do, only by the force of reasoning, for truth will cut its own way.
"Thou shalt love thy wife with all thy heart, and shalt cleave unto her and none else."