I think "prior contamination" would come into play in both "The Omega Glory" and "A Private Little War". In the former, Kirk merely communicated what was already a written document. In the latter, the Klingons were giving a technological leap to one group of people. Kirk merely restored the balance that existed prior to that interference.
Yeah, I think that's exactly how the prime directive should be used.
The PD ist NOT
"Never touch anything". It's
"Don't f-ing push your own agenda on them".
It basicall steams from a deep respect for 15th century explorers, with the wish
they just weren't such assholes to everyone they met and wiped out their civilisations.
Also, I don't believe for one second that only the Federation has the PD. It seems to be the lowest common denominator between all (Alpha quadrant) space-faring civilisations not to meddle with lower ones. Otherwise the klingons would have invaded a shit ton of lesser species and strip-mined their whole planet. No, the only time we saw that, the klingon tried to do it stealthy (with those muskets), pretending the natives developed them on their own, for not to get openly seen violating the prime directive. That implies
even the hostile klingons follow the PD (at least officially). And this is basically everytime Kirk "violates" the PD: After it
was already broken, to try to nullify or at least reduce the impact of the previous breach. The Dominion in the Gamma quadrant on the other hand never seem to had one, and were happy to enslave and meddle with each and everyone they met.
And yeah, we saw the Federation (and klingons for that matter) trading, making diplomacy, and trying to influence other species through political and economical means, or straight up make them members of the Federation and include them in their political system. The PD basically just prevents alien races straight up invading non-warp civilisations, holding them at gunpoint, and tell them what to do.