For that matter, if Eb falls.. wonder what the Blackstaff could do if IT could tap the Demonreach ley line ?

I think it could, but question then becomes would a binding have to be accepted by the island first or not?  Like I believe Harry could use the Blackstaff to tap the ley line, but could someone else not a Warden of the island?

You're getting your timelines muddled there. The coin was under his lab before PG and it stayed there until the end of book 9 (WN). In book 12 (Changes) it has been several years since book 9 so it would be exceedingly unlikely that Forthill hadn't passed the coin along by then.

My timeline isn't muddled..  You are right it was White Night when Forthill got the coin, however it isn't a stretch to think the coin might sill be locked in his office as of Changes.. We have no clue as to how frequent the coins collected by the Knight are picked up..  We do know it was Lasciel whispering to Harry in Changes which speaks to her coin not being very far away, that is my point.

I assume all their magic draws on hellfire including Marcone. But none of them could have thrown hellfire at Arctis Tor without help.

You are ignoring the fact that Namshiel is a fallen angel, even if he isn't an archangel he can wield power beyond the comprehension of most.. So yes, Namshiel could do it without help in my opinion, angels without restraint [the Fallen don't play by the rules, that is why they are fallen] has the power to do it without help.

Abraham was told to present his son Isaac as a sacrifice. An Angel stopped him and a ram was used instead.

Yeah, and Andruiel and his boss could care less about human life as long as they achieve their goals.  That is a difference between those who have fallen and those who have not... I wonder if Nic will begin to ask questions to that effect?

He turned Lasciel in after White Night, the book after Proven Guilty, she then took it up some time (implied to be a few weeks or so) after Changes.

After Changes or at the end of Proven Guilty?  Just before he turned the coin over to Father Forthill at the end of Proven Guilty he thought he heard Lash, that scared him so he got rid of the coin.  In Changes it is implied that it was Lasciel that goaded him into suicide, the coin still could have been locked away in Forthill's office.  I don't think she affected Harry after that, implied or otherwise.

That being said, if you look at Harry's darkest moments (particularly the ones where he tried going down the left hand path - e.g. Storm Front when he's outside Sells house & Battle Ground when he tried to kill Rudolph) he looks pretty bad. Really bad. If you look at the language he uses he very much could be that being.

Yes, but in Storm Front if you are talking about when he was outside of the lake front house that Sells owned, I think he was thinking of what is possible verses him going down that road himself.  Yes, he was tempted for the moment, remember he'd slipped on that banana before when he killed Justin, which makes slipping again easier. But then the spirit of his mother saved him, he felt her touch and he clasped the pentacle that came from her, it reminded him of the danger and what he should be about. 

I don't count his feelings when he went after Rudolph when he killed Murphy as the same.  I think the wild range of emotions he felt in that moment can be considered normal, ugly and violent to be sure, but a pure emotional reaction.  Having said that, yes, I know he is a wizard, Winter Knight, which ramps up whatever reaction he was having, and that makes him very dangerous.  Not unlike someone who is armed and witnessing a loved one being killed.  I've read of at least two war time accounts where upon witnessing "buddies" being killed a soldier goes a bit "crazy" taking out numerous machine gun nests etc single handedly.  However that doesn't make him a antichrist or the antichrist.  Now I can be wrong, but from what little I've read, Harry doesn't fit that definition, at least not yet.

From what Basil said;

So this isn't just a pseudochristos, but THE Pseudochristos.  The Man of Sin is a rebel and a destroyer, an agent of chaos whose hubris and arrogance sets himself above God.  That's what Harry is telling us he could end up being.  And, given the amount of personal power at his disposal .... not an unrealizable goal.

I can see Harry being sorely tempted because he does have what some would call "godlike powers."
Harry doesn't have that kind of hubris.. However I just came across something very interesting when I looked up the definition of "hubris."

(in Greek tragedy) excessive pride toward or defiance of the gods, leading to nemesis.

Is it a coincidence or is this what Jim is driving at by calling the Enemy, "Nemesis"?  Or more to the point, Harry is the one to defeat Nemesis because he lacks that kind of pride and defiance?

I don't necessarily believe it was Malcolm's genetics that played a key role. Rather, it was his parenting. His teachings and modelling of behaviour.

That too, and I think Margaret saw that in Malcolm, it is almost a given that at some point they soul gazed, though I have no proof of that.  She also saw his goodness, there is a reason why over the series Harry is repeatedly told he inherited his father's good heart.  That good heart has prevented him from going too far down the wrong road when he has crossed the line.

think there is an alternate version that could be the Pseudochrist. Whether the Harry we are familiar with could be is another question.

The temptation is there, with the exception of the Gail, he now has in his possession all the Artifacts we associate with Jesus and the Crucifixion, redemption, salvation, however far one wants to go with it.  So if Harry wanted to set himself of as a Christlike figure and demand a following based on that, he now has the tools.  Actually in Battle Ground when ordinary people followed him under "his Banner,"  if Harry was going to go all pseudochrist on us, that would have been the time.

Uriel gave his Grace away. Maybe Raphael did too, knowing it'd be a lot longer before he got it back. A.K.A. the end of the world.

Uriel loaned his Grace temporarily to Michael a Holy Knight to complete  a mission, that isn't the same as giving his Grace away.

Yeah... that's my point.   
Nic isn't working for Nemesis.  By attempting the heist in SG, Nic is acting against the goals of Nemesis.  The outcome being bad for Nic personally isn't really relevant.

Oh, I think ultimately it will be relevant, if it sets Nic on the road to redemption, he may quit the idea of apocalypse and become a valuable ally for Harry.. That is if Harry can accept the idea that Nic is redeemed..  Actually that is how I think it is shaping up, his daughter killed, Harry handing over the Grail to him, yup, look for Nic to join Harry against Nemesis.