In this context, “pouvait” and “aurait pu” convey very similar meanings. The only nuance I understand (quite subjective) is similar, in a weaker form, to the nuance between “he comes at 8” and “he is coming at 8”. The first carries a “constant” sense, the second refers to a specific time.

To me, “aurait pu” might refers (to the narrator) to something special. For instance an event, potential recent, that has affected the place recently, which could have modified my perception of its peacefulness.

To be more specific, I could use the first sentence if I travel for the first time to a country at war since 30 years. I only know the country through the standard news, which periodically shows up as not peaceful. But arriving there, I am surprised of the peace of the country.

The second sentence might arise if I travel to a city that just made the news with a recent outbreak of riots throughout the city. The dramatization of the media made me await a state of havoc, which I just do not see when arriving.

Not much difference in this particular context, but it’s not a general equivalence at all, and the second sentence is understandable but the first sentence is clearer in my opinion.

First off, take away the introductory clause and the meaning is quite different:

Ça aurait pu être paisible

= It could have been peaceful [but it really wasn’t]

Whereas

Ça pouvait être paisible

= It was sometimes peaceful / It was possibly peaceful

Here, with the introductory clause, what would be understood by most people is that the speaker is expressing disbelief at the actual peacefulness of the place, in both cases.

However if I ask myself about the difference between

Je n’aurais jamais cru que nous pouvions gagner la médaille d’or.

I wouldn’t have believed that we could win the gold medal. [and we did]

and

Je n’aurais jamais cru que nous aurions pu gagner la médaille d’or.

I wouldn’t have believed that we could have won the gold medal. [we didn’t, but we had a shot]

then I find myself believing that your second sentence wouldn’t have the same meaning without "aussi". It’s arguable that without it, the sentence means "it’s not peaceful now, but the fact that it could’ve been peaceful is unbelievable." However that interpretation doesn’t work because of "aussi", which implies that the place is indeed currently peaceful, (oh so peaceful).

This ambiguity is why I feel personally like the second sentence is borderline incorrect and the first sentence is clearer.