Most of the time I just describe what the NPC's are doing during combat, and I take away some of the enemies the players would face otherwise. I am not a big fan of all these additional rolls - they can become quite boring for the players if you have a lot of other NPCs involved.
Yeah, I wouldn't ever at the table try and control more than 1 friendly NPC. If there are multiple I'd use the rules for helping so the friendlies would act as a team. Even if I don't want to delegate full control over the NPC's I'll usually delegate their rolls unless I specifically think knowledge of their stats is going to be detrimental to the game. And if NPCs are around but don't directly impact the PC's they all get abstracted away into the narrative.
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, a Forbidden Lands AP which focuses on learning the game, playing Raven’s Purge