I vividly remember needing to spam /pet back off and /pet attacking a new target, to get a pet to switch to a new target. If an npc is actively swinging on your pet, it should be mildly difficult to get it to attack a new target.
I believe scenario 1 from OP is accurate and if that changed, needs to be reverted.
edit: Spamming /pet back off | /pet attack wasn't necessary. If you could time it between npc swings.
If 2 npcs are both attacking your pet.
-both npc A and npc B are attacking your pet.
-Pet is attacking npc A
-if you can /pet back off, target npc B, and /pet attack before npc A aggros again (swing, dot tick, etc), pet should attack npc B
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