I don't know what is being referred to from the last two episodes, although one could argue that not turning up to work, or even calling in, when Rikki stole his gun 3 episodes ago wasn't at all professional, even if you think that it is excusable considering the circumstances.
I seem to remember him mentioning in the episode following Ruben's death that he didn't want to be 'looking in schools for murderers right now', or something to that effect, I think he may have mentioned Ruben on another occasion, but whether it was him bringing it up, or someone else asking him about it, I don't know, and I don't personally think it's worth dwelling on.
I don't think that Lindsay walking out after her so-called monologue was due to her professionalism. She was pissed off, angry and upset, and had just said something that, for whatever reason, she probably didn't really want to end up discussing immediately with him, I don't personally find her behaviour to be particularly strange (but then again, I'm a very unreasonable wife, and often get my own way by being pissed off and making my husband apologise even when he hasn't really done anything wrong. Immature and childish most definitely, but it's an effective mechanism for getting my own way and sometimes I do it just because I'm in a bad mood).
Or, maybe she decided to concentrate on the missing child? They both should have focused more on that and less on being annoyed at each other, regardless of who started it. And it was the timing of that scene that grates on me most, because they really did have better things that they should have been doing.
As for the question in this thread, did Danny cheat? I still don't know, and won't unless further information to either confirm or deny it is provided. I won't be judging him negatively based on what is currently known because it was entirely unclear as to whether or not him and Lindsay were actually dating at the time, I'm inclined to think not.
Danny is a flawed character, one who doesn't necessarily think before he acts. Although I don't believe that he impulsively jumped into bed with Rikki by any means, I don't find this situation at all surprising for a character such as his. I also don't see Danny as being someone who deliberately hurts people. He's an idiot, yes, but his intentions are usually good.