No I haven't forgotten it.  But  Lasciel isn't Superwoman.  Harry defeats her fairly easily in Skin Game.   And Harry would have been up against Morgans sword which is designed to cleave magic and against the Merlin, who by Jim's word is the most powerful living wizard.

Everyone was locked out with the wards the wardens put on the meeting place, Michael couldn't have gotten in until the fight was over.

If the first, Michael is traveling with half the Senior Council and they are followed close on by whoever was chasing them. It's hard to see how a fight would evolve out of the chaos of his arrival in this situation.

In a close quarters fight Harry and Molly would have died quickly. That seems to be established.  Harry wouldn't have had time to take advantage of whatever Lasciel could have offered.  Reason demands that Harry would have had to make the decision to use either the coin or Mab's offer before entering the circle.  And he didn't.

Harry has a rather low opinion of himself, I think. In Dead Beat, he works with Lash after he abandons hope, because he doesn't believe that TWG would think he was worth saving. The glimmer of faith he had was only because there was an ex-Denarian there. Harry never thought that TWG would care about him, only that He would naturally want a Denarian stopped. He doesn't think of himself as a good person.

In Proven Guilty, the person Harry respects most—and what Harry probably considers to be the pinnacle of human goodness—has his family at risk. Because he's such a good person, and because he's pretty much seen that TWG does like Michael, Harry has faith that TWG wouldn't let Michael suffer. That's part of the reason he's so furious in Small Favor, I think.

Then in Changes, Harry no longer cares about temptation, really. He's gone full Winchester-in-the-seasons-when-Supernatural-wasn't-awful, ready to do whatever it takes to protect his family. He'd still prefer a less mustache-twirly option, which is why he goes to Winter, but he'd do it if he had to. In fact, I'm kinda surprised he didn't go for the Coin, too. Winter Knight=strong. Winter Denarian=Stronger.

Hell, having Lasciel back in his head might be helping keep back the Winter Knight's baser instincts right now. Lasciel was more articulate, and I doubt she'd have stood for some of the more primal urges the Mantle pushes on Harry.

Harry found multiple items on the table.  Nic specifically wanted the grail.  He killed his daughter and engineered the raid for precisely that purpose.  Did he know of the other items in any specific way?

I disagree with this "what Lash should have done" routine.  If Harry was any other moral - ANY other mortal that Lasciel had seduced in the past - this would have been the knockout blow.  Lasciel is presumably intelligent enough to have deduced where this trip is ending up.  Running Little Chicago is possibly the least dangerous of all the things Harry is about to attempt, and it's going to get his head blown to pieces.  Lash *knows* that she's not going to get anywhere with the "let me slowly guide you" routine - she NEEDS to offer him the coin.  She does his best attempt, and he refuses - and by doing so, accepts that he may die by his decision.  There's nothing else that she can offer.

The situation you pose is if suddenly there was a battle that broke out in the middle, when the Merlin declared Molly guilty.  Yet I don't see this as being any more hopeless of a situation than storming Arctus Tor, or fighting Eldest Fetch - in that they are completely hopeless, save for information that Harry doesn't have - that he has Summer Fire, or that all of Winter is away.

Lastly...  I feel like you're completely forgetting about Harry's last gambit - that this is, in itself, a leap of faith.  Harry places his faith in TWG, even if he isn't willing to really admit it to himself, that either he had the power to bring Molly back or that Michael was somehow involved and would appear.  When every situation got grimmer - learning that the Merlin had all the votes, that his defense of Molly was over-reaching, that the Merlin pronounced Molly's death - he got more resilient and audacious. 

I mean - there's absolutely NO reason AT ALL to demand that everyone must wait while the Gatekeeper makes his last vote.  The chances against something suddenly changing in the few minutes it would take would be trillions to one.  Yet Dresden DOES demand this, and they DO wait...  not because this is the only option available to him, but because Dresden actually has faith that TWG will pull through.

To think that things might suddenly get a bit spicy, and look dire, and Lash might do her time slowy thing, and Dresden might think, "You know, screw it, this faith thing seemed like a good idea, but I might as well turn into a monster after all to rescue this kid who CERTAINLY will turn into a monster without proper guidance, which I can't provide anymore - THAT is contrary to his character.  Because the more dire things look, the more certain Dresden is that those doors are going to burst open and salvation is on the other side.

Why?