Worf's foster brother was not Starfleet IIRC, but an independent Federation individual studying a pre-warp civilization. As also are the duckblind operators in "Who Watches the Watchers". The suggestion always seemed to be that they were meant to be bound by the law of non-interference.
It's referred to as the Prime Directive and Starfleet's general order number one. I don't think it's referred to as Starfleet's Prime Directive, so it could be the prime directive of the Federation. It is probably a case of upholding the Federation's Prime Directive being Starfleet's general order number one....
Apologies if I'm just rambling rather than making any sense.
Homeward is such a horrible mess that I'd rather tend to discard that episode. If I took it seriously, extinction of a species being preferable to interference to at least
save said species, and deal with 'cultural contamination issues' later would be an acceptable interpretation of the PD in Picard's time -- and I won't accept
that anytime soon. (And even contamination wouldn't have been necessary -- just sedate them, put them in stasis and transport them) . But OK, this is just an aside rant, and not really relevant to the argument

.
That aside, it
could be proof of the PD extending to other groups as well. However, mem Alpha says Nikolai was assigned to a 'cultural observation post', and we don't know under whose authority and charters such observation posts would fall. Could be that even though they are run by civilian-looking observers, they might still fall under a branch of Starfleet. Or any other organisation with their own rules.
In
Who watches the watchers we see that the group of scientists
themselves try to keep up their cloak as best as they can -- but I don't think the episode implies that they are
formally bound by the PD -- though they could be, there also could be a (formal or informal) honour code among these researchers against cultural contamination, being very similar to the PD.
Also, it could be the case that the PD applies to more groups (e.g. scientists at government institutions in general), without applying to every citizen.
What about those who aren't part of the Federation? How do we explain their (apparent) non-interference with pre-warp civilisations? We rarely see or hear about them plundering pre-warp planets for resources.
Are we to assume that there is a galaxy-wide, prime directive style agreement or is it the case that we just don't see the Romulans mining the shit out of a planet as the local cave men sit and watch in confusion?
Well, in VOY there were the two Ferengi in the DQ. VOY interfered with them. Would be interesting to know what had happened had they tried their little scheme somewhere in the AQ or BQ (but outside Federation territory).