just for the sake of discussion.

First, I don't think that Denny and W. were particularly close friends - IIRC, Denny didn't go to London on purpose to get Wickham, he went on other business and while there he by chance "renewed a slight acquaintance." From this slight acquaintance, W. hinted or otherwise weaselled a recommendation to the regiment.

From my knowledge of the camaraderie of young military men, which does not always depend on intimate friendship, and from my own opinion of W's manipulation skills, I had formed the following impressions -

The prelude to the Netherfield Ball plays in my head as follows:

Denny: Wickham, why aren't you dressing for the ball?
W: Oh...emmm...well...I'm not going. I...have business in London and must leave this evening.
D: You never mentioned this before, it can't be that pressing!
W: well, I will confess that I have my own reasons for going tonight. Mr. (pause) Darcy and I perhaps should not meet in public - I cannot in conscience tell you why. But, would you be so good as to convey my deepest apologies to Miss Elizabeth Bennet? I am sure she will understand entirely.

Since Darcy had not left yet, and Denny had no strong reason to think ill of him, I doubt W told his "bad Darcy" sob story just yet. I think that merely hinting that Miss Eliza would know the story might have excited Denny's curiousity, and caused the significant smile, as young men often enjoy appearing "in the know" when there's a great secret afoot. (As, of course, do many young ladies. I've no wish to be unfair.)

As to his "slip" that Wickham had no honorable intentions towards Lydia, it may have been no more than the following -

Random gossipy person: I say, Denny, did you hear the news about Wickham? They say he's fallen mad in love with Mrs. Forster's young friend - he's run off to marry her!
Denny: (laughingly and disbelievingly) You're out of your mind, man. He's no more mad in love with Lydia than I am with my horse - and he certainly isn't planning any marrying that I know of! He'll be needing a wife with money eventually, you know that as well as I.

Later, when confronted by Colonel Forster and realizing that RGP wasn't just whistling Dixie, he may have backpedalled a bit:

D: n..no sir, I mean... I never said he wasn't going to marry...I...I mean...I never heard him say anything about her at all, that's all. I don't know where they've gone or what they're up to... I don't know anything about it at all...