“It would be nice if the North’s nuclear-armed government really were unstable, less because the world could then breathe easier, more because North Koreans could then breathe freer.”

Should the correct form be:
“breathe more easily” and “breathe more freely”?

Hello, Rajam-- and welcome to English-test.

No. The informal use of the adjectival form as an adverb is quite acceptable nowadays-- at least for these two.