@10ebbor10 said the point I was going to make quite well (possibly better than I could).
You are making a lot of assumptions here; I will try and go through them but by bit.
1) Peace will not last; Urth will turn them against us.
This is a self defeating prophecy. If you believe peace will not last and thus never attempt to achieve it, then of course it will be true.
2) The Caradysh have the ability to consistently poison the lowlands against us.
Even if they do, it would mean that they have to tangle up some of their actions consistently poisoning the well instead of being able to use our existing shitty reputation (Rich Mountain Demons) in order to get people to easily attack us.
Here is also a nice quote from
@Hallesworth49 :
I'd like to add to this that the diplo action could also serve as an early warning, on whether our relations with other civs take a turn for the worse and thus we can be prepared for either war or to avert it.
Another quote from
@Kiba
Diplomacy also keep us abreast of technological innovations happening in other areas, and let us get useful animals and goods.
3) Attemtpting to gain peace while the Caradysh exist is pointless and thus we should not do it.
This is demonstrably false. Peace means that we don't have to worry about getting sneak attacked, it means that we can finally have an information source in the lowlands, it means that the Caradysh have less people to throw st us in combat, and so on.
4) The Lowlanders are a monolithic block that have already become under the sway of the Caradysh.
This is also false, as we've just seen with the Maradysh. No civ is a monolith (including our own highly collectivist civ); we will be able to find someone who support us/doesn't think we're terrible if we work hard enough at it.
Overall, the vibe I'm getting from you (and the anti-peace people in general) is that diplomacy is not worth the effort.
Diplomacy is always worth the effort.