And, yes, the PD sounds simple in principle, but gets messy when you try to actually apply it to an infinite number of scenarios.
And the more you think about it, it is ultimately futile to the purpose of preserving cultures, when they show no signs of actually doing anything about it if someone else is disrupting them
If you couple that fact with the odd appearances of Starfleet intending to allow species go extinct to avoid a PD violation, the PD really begins to look like a very self centered policy. "WE don't want to get caught up in a mess, so here's a rule that says leave well enough alone"
Stopping third party interaction.
There would have to be reasonable limitations on this
OMG, so many that it would be a bureaucratic mess, which is why I don't really recall seeing it happen on the show. Just too many variables to have a stipulation for. My only point is that it's the moral thing to do if you're saying your bedrock principle is to preserve a culture's natural development
It would depend on whom the PD applies, my take is that it applies only to Starfleet, and isn't something that the Federation expects other species will adhere to throughout the universe.
Yeah, they're never shown pushing that agenda on others. It's ultimately just their rule for how they want to handle these situations themselves, in a manner that lets them walk away guilt free, and if someone else comes along 5 minutes later, and does the very thing they prevented themselves from doing, that's none of their concern. Shit happens
There's a implication in the very essence of the PD that the Federation inherently doesn't trust it's own people.
Of course they don't, because what they're asking is essentially impossible. When you begin interacting with a people, you alter their development just by your mere presence. Trade relations alone can have the capacity to do that. I can see having a directive that prohibits messing with a primitive culture which doesn't even know about the galactic community yet, or is so underdeveloped that your interaction might throw their whole way of life off balance etc...
However, this notion of a PD that stops them from getting involved in a Klingon Civil War, when evidence shows Romulan involvement, or involves species that they have longstanding relations with, it's just ridiculous. It's basically a giant blind eye & deaf ear trump card they they play, that serves no purpose but to let them off the hook on galactic geopolitics whenever they want.