I mean, I'm a fan of time traveling Harry doing it
Maybe he did.... Or maybe he was the one who showed up and stopped it.
Marcone didn't like the odds in Battle Ground and declined and it cost him the Eye and his castle.
Well I don't think he realized his castle was up for grabs at that point. It did cost him the Eye but I don't think it had to do with the odds against Harry on his own. Harry did have the Spear of destiny with him, and it may be on par with or more powerful than one of the Swords. If Harry had been without it, who knows. That being said Marcone did say that he had a more longer term perspective. Perhaps it has to do with Harry the Starborn. Maybe he knows Harry is going to be needed, or see's him as a tool for later, or that ticking off Mab wasn't worth the trouble.
Marcone/Namshiel have no real shot at defeating Harry in a duel at present.
While he has shown some advanced defensive magics, he has not demonstrated a single offensive ability beyond enchanting single-shot muskets. Unless he's got a lot more hidden away, Harry can laugh at his attacks behind his shield and wear him down.
That's because Marcone keeps his abilities close to the vest, he doesn't boast. He's had the Coin for years and just now showed his cards because he had to. He didn't use offensive abilities because they wouldn't have been effective against a Titan, and would have been a waste of time and energy.
We know that Fallen can throw around Hellfire, a devastating, destructive force. At the very least Marcone has that. Also he broke down concrete at the molecular level. What do you think he could do to a human body?
"I noted, somewhere in the academic vaults of my head, that magic like that was like unbaking a damned cake back into its original components" "Ramirez was good. Better than me, on a technical level, by a considerable margin" - Harry
"I just broke down the molecular structure of concrete and then chemically re-formed it inside a mold of pure will" - Marcone
One could argue that Marcone not only is capable of what Ramirez did, but by re-forming it is beyond Carlos's abilities. Carlos broke down the molecular structure, we haven't seen him reform something on that level. Destroying is easier than creating.
That's not to say he won't become a magical threat. It just makes more sense for him to learn defensive magic first and rely on his army of troubleshooters for offense. Once he feels he can adequately defend against one or two Wardens, he start learning offense so he can be more self-sufficient. He only revealed what he had because Ethniu was an unexpected situation and the battle was at the desperate last stand there on the beach.
Displayed in some of the short stories, Marcone has been preparing for a showdown with Dresden for many years. His special bullets were created by Gard (maybe he does them now I don't know). He had come up with strategies to defeat Harry like using explosives to wear him down, a sprinkler system to dampen his magic, and the special bullets to go through his shield to kill him... Now he has a Coin, and has shown to be a very capable magic user. I find it highly unlikely he hasn't been learning offensive magic. He's shown to be at an expert level with some magic already. Harry has said that fire magic is easy, and one of the first things a wizard learns. Marcone at the very least could probably burn Harry to a crisp.