"Do all Keys’ dive operators require a guide? What’s up with that?"
They all require a buddy as no DM/guide leads the dive in the Key’s like many Carib islands do so you need to find your own way back to the boat and make sure you know which boat you are on because it’s a parking lot out there.
You can find a buddy at the boat but beware of three somes as neither looks out for the third person :(
There are still some cattleboat operators in the KL, one that I know of will sometimes put snorkelers on the same boat as the divers so you end up with a shallow beat up reef :( Looe Reef Resort in the lower Key’s also does that.
The one that had a history of leaving divers behind is gone!