Harry told them.  He discussed it with Murphy.  Anduriel was listening in.

I don't think Harry had to tell them, depending how well Mab hid the knowledge of it, it was out there for Anduriel to find out about.

Harry's a monster.  Showing remorse doesn't mean anything. A man might show remorse after beating his wife to death. It makes him no less a monster. The day might come when Butter's or someone isn't there.  And then what? And this isn't the first instance of Harry going into a killing rage.

A true monster in my opinion is a psychopath and or sociopath, Harry is neither.  A monster wouldn't have demanded that the people hurt and killed in the battle be compensated.  People lose their tempers all of the time, that doesn't make them monsters.  We tend to cut Rudolph a bit of slack for killing Murphy because he was paranoid, scared shitless, and totally careless.  Yet, many aren't willing to cut Harry a bit of slack, react, yes, but with merely a flood of grief, not violent retaliation.. That makes him a monster some seem to think, but left out in that scenario is the kind of pressure Harry was under at that moment,people were dying left right and center and he couldn't do much about it, then Murphy was killed before his eyes, he snapped, everyone has their breaking point.  Sanya and Butters know this, if they thought for one moment that he was a monster, they would have dispatched him, the Sword would have instead of a mere slap on the wrist in the form of a burn to bring him back to the job at hand.

Yes, to an extent Nick is manipulating the more powerful Denarians to keep them in line, giving them what they want, or keeping them on ice or marrying them. They are not a homogeneous body.

Thorned working with Marcone is a case in point, I suspect Nick gave Thorned weak hosts, and kept him on ice and isolated when he didn’t need him for the big stuff. His time with Marcone where he has been out of the box and had a strong host who cooperates with him like Nick and Anduriel, May be the best time he has had in 2,000 years.

I agree, then again, Marcone is also strong, if their agendas don't mesh, he is strong enough to resist Namshiel.

In Battle Ground Harry came as close as he ever will to being a monster, that’s why Sanya and Butters were there, not to kill a monster (the Titan) but to stop the birth of a worse monster (Harry).

I don't think killing Rudolph would have made Harry a monster.  Now if the attempt had been unprovoked, coldly, and more importantly for pleasure, then he would have become a monster. 
Don't get me wrong, killing Rudolph still would have been bad and when he came to himself again, something Harry would feel guilty about for the rest of his life.  But in reaction to what he saw as the murder of his friend and lover?  Not a monstrous act at all.

 Talent maybe at the OP level, but they'd never make wizard, sure they got some juice from the coin, but the only real magical bad ass in the bunch was Namshiel.  Ascher could have been a wizard but she went warlock instead due to making some bad choices early on.  Thus like Molly she was on the run depending on her half turned friends, when Harry did in the Red Court, most of them died because the mortal halves of them were very old. That made her bitter, take up the coin, and Lasciel powered up her fire magic more than it was.

  Again, is it a real curse?  We know what Malcolm told him... When he was mostly dead, he fell from the boat into Mab's waiting arms.. Yeah, he heard in his mind Cassius's death curse, but he was only mostly dead, Mab was there so even if he was all dead, he didn't die alone.  If it means that all his friends and family die first, well, if Harry is the military leader of the BAT, that stuff happens...

One more thought, was Cassius even a wizard?  I think not, so there was no juice behind his curse other than to make Harry feel bad.

The return of the Blackstaff could win Harry an unravelling, which could cure Thomas of his demon, something foreshadowed with Susan

That could work, but would Thomas want to come back if Justine isn't there?

I think a Dragon would trigger Mab calling back not just her guard, but some of those 40,000-faerie-strong formations round about the outer gates.

And at 10's of thousands plus Mab to one, it might not be so easy, especially when the Mothers take notice.

Or a Holy Knight or two, let's not forget that Michael killed a dragon to save Charity.