as well as the emergency helicoptering in of another detention centre to service Ganks2
The local detention centre is necessary to prevent a fast-travel exploit where someone could instantly travel from Sag A* back to Colonia by committing a minor trespass or assault offence, sneaking back into the station, and handing themselves in.
(It'll also stop the inevitable complaining when someone doing one of the CGs accidentally acquires a 100 credit bounty and would otherwise have to go right back to Colonia to clear it before they could contribute to the CG)
and a proliferation of pirates in the zero population system that houses the still-born carcass of Explorer's Anchorage.
750,000 population system according to the map. Maybe people could do the bounty CG by protecting the miners with supercruise escorts - as the description says it's for - rather than by going and shooting up some pirates a long way from anything relevant to the mining CG?
Why not have the Detention Centre in the same system as the station
There are a number of special properties of detention centres that mean they work much better when not in an otherwise occupied system - every single detention centre is in its own system. (Among other things, it means they're easier to isolate from the effects of the BGS, can have a security level to discourage fighting in the detention system itself, and the systems can otherwise count as unoccupied.)
Why not have a shipyard with no ships or modules to buy so that commanders could at least call out their specialised mining ships for the CG?
You complain - wrongly, in my opinion - about the Detention Centre being there for DG2, and now you want a way for every ganker in the galaxy to be able to transfer their 10LY-ranged combat boat to Sag A*?
Also, it's not as if this CG hasn't been advertised two months ahead as "a mining CG at Sag A*" - and the outfitting allows any ship to be refitted for mining purposes, with many common exploration hulls making excellent miners. If you want a CG with bubble-quality facilities and convenience ... do them in the bubble?
I've made a few runs tonight and not had an NPC jump into my instance once. I'm using the bauxite hotspot at Juenae AJ-C C2-1813 2 (9.61 Ly from XA, 571 Ls from entry) and carry a couple of Guardian Orbs for recreation at meet-ups. Normally having something in the hold like that would bring NPC pirates swarming, but not a thing. I think it's because all NPC activity seems to be limited to the system Explorer's Anchorage is in.
That's likely - only Sol and Colonia have the bubble of activity for uninhabited systems.