You have not answered my question. Do you in general violate people and force them to do your bidding because you think you know what's best for them? I hope you proceed differently, explain them the danger they are in, offer your help without applying any force and don't mess with them if they don't want your help.
You obviously either haven't read or haven't comprehended what I've said...
If you have the belief that they are a prewarp culture, the Prime Directive would prevent you from even offering them help.
Any decisions you make comes down to how you value those six hundred lives and what resources in people and technology you're willing to use to that end. The Prime Directive simply doesn't fit the situation and I hope it's adjusted after the fact to reflect the new reality the situation represented.