Can't be a plea deal when the accused didn't even place a plea or try to make a deal. V. was truly ready to release Dantes, and Dantes was ready to head out the door. Until V. found out the letter was addressed to Noirtier. Up until that point, V. wasn't ready to prosecute Dantes (and he wanted to impress Renee with his mercy). V. had seen real foaming-at-the-mouth Bonapartists and wouldn't hesitate to condemn them but that boy wasn't one of them. He saw a dumbass who was in way over his head in something that he didn't truly understand.

V. didn't hang out with the old man because of political ambitions. But we have seen that V. was willing to warn Noirtier that the police were looking for Gen. Quesnel's assassin, and the description of the perp looked just like ol' Dad. And V. just stood by as Noirtier shaved and changed his clothing, and even took parts of V's own wardrobe. So they had differing politics, but V. wasn't hostile or willing to let his father get arrested.

Yeah, Dantes was a naive fool, but I'd write it off as "typical 19 year old". Dantes might have heard a few things here and there, but blew it off as, "Meh, doesn't concern me. No politics for me. All I care about is my job, my Dad and marrying Mercedes. Oh, the captain's last wish was to deliver this letter... ummm, OK."

And that's exactly what he told V. and V. saw his sincerity and believed him.