This was dialogue from a detective manga where a police officer is interrogating a suspect.
(I got intimated with just the of us alone (?))
(I'll force through this)
I did find this https://thesaurus.weblio.jp/content/口を割らない so I assume it means to "keep one mouth's shut" Is that correct?
But my main issue here is I don't know what もののなかなか means here. Is it an adjective here? There's also a lot of different meanings for it so I'm confused.
Japandict says it means "by no means, not readily" when using with a negative verb. Though in that case, I'm not sure what the function of "ものの" is.
Is it "They won't reveal the truth by any means"?