To be clear I said "If someone who is not downed has healing word one PC getting downed doesn't really even reduce the parties overall effectiveness."
I get that nuance was easy to miss and arguably off topic to the post I replied to..
Having 2 down will reduce effectiveness, because it is going to take the healers action but not by a large amount if you have a healer cleric because that is all it is - one action. Aid usually brings back up to 3 characters on a single 2nd level casting and it upcasts well. Aura of Life will bring back every single downed PC over and over again at the start of their turn (i.e. no lost actions) as long as they are within 30 feet and the Cleric maintains concentration.
Using your chart:
Monster downs 2 PCS
Aura of Life - 3 APM (not counting Cleric since he used his action on the casting)
Monster downs 2 PCs
2 brought back from AOL - 4APM
Monster downs 2 PCs
2 brought back from AOL - 4APM
AOL lasts 100 rounds if they don't break concentration. Add sanctuary to all this in round 2+ as a bonus action after the Cleric makes an attack or cantrip.
As long as the party has healing this is going to go on and on and on. If they target and down the Cleric and other party members have healing they bring him up and he brings up the other downed guy and we keep going.
If the monster downs all the healers, then yes the fight is lost.
If you are fighting with multiple enemies who make melee attacks, the best course of action is usually to attack a downed PCs instead of downing another PC. A melee hit from 5 feet away on a downed PC is automatically a critical and causes 2 failed death saves, so 2 hits on a downed PC kills her outright. If you have multiple enemies or multiple attacks, this is usually the most effective strategy to counter yo-yo healing.