Because to their knowledge, nobody had ever resisted the temptation of a fallen. Harry being who he is was just stubborn enough to resist the call of the fallen. To the church's knowledge, harry is the first to resist the temptation. Though had Harry not had the wonderful conversation about how Lasciell's shadow was composed of Harry's fungible material it is uncertain if she would have changed... she resisted right up to the end.

Sonja, a KoTC had given up a Coin.  Others have given up a Coin.

Or because he's been targeted and someone sacrificed themselves for him already, so he can't be targeted again?
*Heck, everyone talks about Cassius's death curse in relation to Murphy, but Nic could have done it too. Small petty revenge from whatever hole he's currently hiding in.... That.. might explain why the knights showed up in reply. They were on duty there.

In Book 5 when Harry gets hit with the Barabbas Curse and Shiro takes it for him so he doesn’t die, what stops Nicodemus from just doing the Barabbas Curse on Harry again?

JB: It takes a while to set it up and get going.  You can’t just whip it out and say here’s a curse.  There’s got to be blood sacrifice and ritual and all this other stuff that goes along with it.  Otherwise he’d just use it every day, for fun.  That guy cut me off in traffic.  Barabbas Curse.  Because he totally would.

I'm also almost positive at some point Jim said that because someone sacrificed them self for Harry, that it can't be cast at him again.  Not positive though.

It would be nice if they let him in on the plan..

Well to make something convincing you would not let him in on it until the last second.  So his actions are totally natural.  It also allows them to monitor him to see if he actually is Black Council. 

Who are his friends on the Council?

McCoy
Gatekeeper
Listens to Wind
Martha Liberty
Lucio who is either captain of the Wardens, or near the top.

I could see where McCoy and the others are using the Councils fear of Dresden to their advantage.  Let the Council kick him out, and then monitor if someone tries to contact him.  The Grey Council fears the White Council has spies.  So allow Harry to get kicked out, and see if someone tries to contact the Starborn, Warden, Winter Knight, and holder of powerful weapons.  I mean how could the Black Council not enquire about Harry as an ally with all that power?

  Just a thought, don't have the exact quote, and I am not even sure that Harry could undo it.  I think just before Harry tries to kill Lea with a Holy Sword and loses it.  Lea expresses interest in Michael's first child, and that would be Molly..  Ultimately Lea becomes "Auntie Lea" and Molly is prepared for becoming a Lady at some point.  It happens in Cold Days, and in typical Mab fashion she lays the blame at Harry's feet.. In Peace Talks he apparently agrees, then in Battle Ground Mab orders him if she is killed to kill Molly..  So a boatload of guilt here for Harry to heap upon himself both earned and imagined.  Not just the prospect of having to kill Molly, but saving her human mortal soul as well, would that be an incentive for Harry to go back in time to fix what he screwed up?

A deal wasn't made for Molly here though.  If he wanted to protect her, I think it would have been in decisions he made later on.  His Choice in Changes ultimately led to Molly being alone, and needing Leah. 

If he could have changed something, not raising the Sword to Leah would have been it, I think.  If he hadn't done that, she wouldn't have had it to trade.  Michael may not have shown up (or he'd have had the Sword to protect himself), and Susan would have been able to use an invitation. 

Marcone will have taken up the shadow and got it to show him its worth, but probably waited years to take up the coin.

I was under the impression that only a wizard can carry around the shadow of a Coin.  A mortal who gives up a Coin will no longer have a connection to it.  Harry who had buried it, and wanted it to go away, was still connected to it.  I thought Michael said something about it to Harry, and how he would eventually be forced to kill Harry, when Harry gave in and accepted it.

So Nicodemus once a year can use his Barabbas curse.  He attempted to kill Dresden with it, but Shiro took Dresden's place.  According to the Bible, the people had a choice.  Release Jesus who had done nothing wrong, or release Barabbas, a known criminal.  The people chose Barabbas.  I suspect that Nicodemus is able to choose who lives, or who dies, once a year.  In this situation he used it to save Blood on his soul who was smashed to paste, and should not have survived.

It had long been speculated that Marcone had the coin. Mab even made a point to look at Marcone during the conversation about what weapons could hurt Ethniu. One thought that I have had is what if Marcone only took up the coin for the fight. If he had however been holding the coin in reserve for such a time it would be in character for him. He has the willpower to talk to and use an imprint just as harry did, without taking up the coin. There is no reason he couldn't have gained the necessary knowledge from Namshiel prior to picking up the coin. As a side note, it is also possible for the fallen to take direct action so long as their host "wills" it, meaning Marcone would not have to have the magical skills himself to use them. Just as Butters was using BOB to power artifacts the fallen could be used to fuel Marcone's magic.

I'm not sure I agree with this.  Marcone was doing really advanced magic.  I'm not sure he could have if he just allowed the Coin in.  I think he had the Coin for a while.