Why don't the two teams ever run into each other or turn out they're Working the Same Case?
They've actually somewhat averted this one on a few occasions. Wheeler's actress was on maternity leave during the filming of the last few episodes of the current season (they wrote her pregnancy into the show to explain the character's absence) so for the episodes where she's absent, Eames is partnered with both Goren and Nichols.
In the Season 6 finale (while Goren is on bereavement for his mother), we see Eames in the squad room. The (lack of) reactions to this suggest it's fairly typical for them to be in the same space at the same time, we just don't usually see it.
The episode "In The Wee Small Hours" is an excellent aversion of this. All four detectives work the case of a missing girl who was in New York on a class trip.
Regrettably, though, Goren and Nichols never worked a case together.
They did work together in the season 9 premiere.
This one makes more sense when you consider that Major Case only gets very select cases, and that the department is small enough that if they got two "separate" cases with a lot in common, someone would pick up on the connection. In order to have a Working the Same Case scenario, you'd have to have two connected cases with no apparent similarities or obvious connection that each end up being classified independently as a Major Case. The odds of that aren't exactly overwhelming. We do see cases where a Major Case ends up being related to something that was being handled by a standard precinct (a murder in "Poison", a missing persons case in "In The Wee Small Hours"), which is far more likely to happen.
One episode revolves around an internet video, the stars of which are kidnapped while filming it. Okay, so who uploaded the video?
They staged the kidnapping so they could upload it. And since the 'kidnappers' quickly posted a ransom demand, it's reasonable that they uploaded the video so people would find out about the kidnapping.
In "Magnificat", much is made of not being able to charge Paul Whitlock with anything, despite the fact that he emotionally abused his wife until she snapped and blew up her car in a murder-suicide attempt, successfully killing three of their four children. Only they had already determined he had taken steps to cover up what she'd done (returning an appliance she had broken in an earlier, ultimately aborted murder-suicide plot, sweeping up the garage where she had made/planted the bomb), so while they can't charge him for what they actually want him to pay for (driving his wife over the edge), they could charge him for covering it up; that's hindering prosecution/obstruction of justice at the very least and could probably be stretched to an accessory-after-the-fact charge — God knows that other prosecutors in the franchise have done more with less. (And in this case, filing charges wouldn't be just for the sake of moral outrage; the father being charged with crimes related to the deaths of three of his children would also give the family court attorney representing the grandmother a pretty slam-dunk case for why the father shouldn't retain custody of the surviving son.) Maybe Carver should have looked at his "book" a little more carefully.
Goren goes into absolute denial when the heart sent to the station turns out to have been Nicole Wallace's. But when you think about it, his denial is justified. Nicole Wallace was the most cunning and dangerous criminal mastermind he'd ever encountered. If someone managed to murder her, cut out her heart, and send it to them, then there's someone potentially even more dangerous out there.
It's justified even more when one considers that if Nicole is in fact still alive, than that means he really isn't as safe and free as Gage (Goren's mentor and the man who claimed to kill Nicole) might think. And it isn't as if Nicole isn't smart or charming enough to trick the right people into changing a few little tidbits of information to fake a death like this. If she's alive, she could at any time resume tormenting Goren. No matter which way you slice it, Goren has every right to be paranoid about this.
Goren being The Unfavorite of his mother who doted his irresponsible, drug addicted brother may be on a more subconscious level due to Goren's biological father, who was a serial rapist/killer, had viciously beaten and raped Goren's mother after she wanted to end their relationship. So, while Goren's mother does love Goren, she also may have been subconsciously resentful to Goren for being a physical reminder of the man who assaulted her.
Combined with this, because Goren is the one taking care of her, he's sometimes the one who has to make the hard decisions that she doesn't like, such as committing her to a facility, while Frank is never there for those. Especially if she already was primed to favor Frank, it would be easy for her to get it in her head that "Frank would never do this to me!", even though everyone else can clearly see that Frank just doesn't care at all.
The ending of "Maledictus" had the publishing tycoon end up being the one who killed his friend, a Mafia Princess who was more likely than not going to be killed by mobsters working in junction with her father, who was sent to prison for life due to her best-selling book and them wishing to silence her. Even with his own money possibly getting him off or at the very least, a lighter sentence, he could easily end up being murdered by the mafia himself.
