1. Tracking turns of in the entitiyClasses.xml, comment out this: <property name="MapIcon" value="ui_game_symbol_bullet_point"/>
It would take a bit of code to make that hired only.
2. Never tried so many at once, try it and let us know.
3. Vanilla NPCs wont work as you said, at this time you cant do what you ask.
1. That's good, it means turning off tracking should be easily done via a modlet. Point taken about the code needed to only track hired NPCs.
2. Well, I tried this, with mixed results.
First, it seems that NPCs de-spawn when you leave the chunk. That's actually not a big deal to me personally, and it actually might make NPC-heavy worlds not consume a huge amount of resources.
Second, I tested this by adding them to many of the vanilla spawn groups. This worked somewhat, but not quite as intended.
Part of my plan was to have NPCs spawn inside POIs along with the zombies. The idea was that you would go to loot a POI, only to find one or more survivors who are already fighting for their lives, and you could help them or not.
They did spawn in these cases, but the NPCs didn't do anything. They just stood there, and didn't fight any woken zombies, or wander around, or anything like that. (They did work if you hire them after they spawn.)
I'm pretty sure this is because POI spawns are sleeper volumes, and the NPCs (zombies in vanilla) spawn in a sleeping state. In this state all AI is shut off. Waking zombies probably involves calling some kind of method that your NPCs simply aren't programmed to respond to.
Zombies would also not wake in their presence, I'm guessing because 7D2D codes them to only wake up to players. That could be OK if the NPCs did spawn in a woken state, because then they could target the sleepers, which I'm guessing would wake them.
So, I think they will only work if they are spawned into groups that aren't initially sleeping. Friendly/hostile animals are in this category, which is why your modlet works as is.
(I'm not expecting you to add the "wake up" code, just letting you know what I found.)
3. That is a pity, but it was a long shot anyway. C'est la vie.