Okay, but easy to get confused because it is also used as a nickname for Carlos.  Why this assumption that defense of the island would be one on one at all?  It is made pretty clear that
though Harry is dealing with a creature whom he name's Alfred.. Alfred/creature and the island are one and the same.. So whoever attacks the island or is unwelcome on the island by Harry, is fighting the whole island.. And the island I think will use any means necessary to combat the intruder or intruders, all at once.

Because we have not seen the island or Alfred engage two enemies at once. He's not a rapid fire machine gun, at least that we have seen. He's the Death Star turbolaser, an incredibly devastating shot - but single shot and reload. He alternated between Lara and Freydis. He didn't hit both at the same time.

I think this is generally true, though maybe individual SC members (lookin' at u Cristos!) might be dumb enough...

But nerds -- including Dresden nerds -- seem like moths to a flame, for "who beats..."  topics.

At a certain point, raw combat power is just not really relevant anymore, it's not going to carry the day:  they all have enough badassery to manage a killshot; it's just a question of who was better-prepared / sneakier / etc... for example, who thought to bring a bagfull of (iron-armed) Little Folk to a Queenfight?

And Harry won that day by having too many attackers to parry. the 7 on the island might not all leave, but one gets Harry in that setting.  If they were going to launch a frontal assault from Chicago, no, I think not.

Well, at the end of CD, Harry told Alfred to take Mab below and lock her up if she killed him. So even if it's one at a time, none of the WC or wardens can handle that.

And yes, Harry can break LOS pretty easily. All he needs is for Alfred to whip up a fog or sandstorm while he runs into the forest. Then he plays hide and seek for the few minutes it will take Alfred to grab or squish everyone. And that's the key. Harry doesn't have to fight, he just has to give the order to Alfred and survive till the action ends. And his worse outcome is a draw. The WC has a chance to get him with their first shot but Alfred will still get them just as he would have Mab.

Oh, no doubt Alfred can jail any of them, I said as much already. But do you think the Blackstaff has never had to track someone in fog? HARRY has done that with Arianna. I don't think that alone is enough. His worst outcome is Harry gets killed. his best outcome is likely that Alfred does manage to get them all before they get off the island. By running to the dock or flying off.

You're making this all about Harry and assuming that Alfred is passive once Harry starts the action. Lara and Feydis may be weaker, but their reaction speed is better than any warden's and they were sunk without a chance to dodge. Harry can have Alfred bury them repeatedly as soon as they pop up. And all Harry has to do is break LOS from his attackers. He knows their exact locations and they'd have to search for him in a forest of actively hostile terrain, trees swatting them with bone crushing force and every animal trying to gnaw their faces off. It's far more likely that Harry gets the drop on them rather than vice versa. And that's the least Alfred can do, we don;t know what the bigger defenses actually are.

Alfred has always been somewhat passive when he is not under order. Remember him letting the fae pound on him? He didn't run Harry off. Yes, Lara and Freydis are faster. And they never got Harry into range of their attacks - they were all hand to hand. So Harry had time. If Harry faced off with the Wardens and SC, he is in range at all times. He zaps Eb and LtW, Mai zaps HIM. He has to take 7 out before they can fire.

Break LOS? First, he has to do it. You think that the Blackstaff never had to find an invisible target and deal with that? Body heat. heatbeat. Echolocation.  If Joe shapeshifts to a bird - he is not ON the island, he is ABOVE it.  All 8 have effective attacks, Lara and Freydis had NONE until they could close.  Alfred seems to be one at a time. Why didn't he scoop up  Corb and a couple of others when he got Ethniu? Because he does one thing at a time. Yes,  harry could fort up, go down into the cell block before the SC arrives, lock the door behind him, and tell Alfred to chase anyone who comes on the island away, and then let harry out. But that is not the same as fighting 3 Senior Council and 4 Wardens likely more on Morgan's level than, say, Carlos.   

I think harry could hole up, and set a perimeter and be very hard to root out on the ground, and impossible in the jail, but once they get on the island? haven't seen an ability to take out a lot of targets. Powerful ones, yes, even Mab, but in small numbers.

   I doubt that it would be as you describe now that there is a Warden in charge and Alfred can fully engage the island defenses.  It is a maximum security prison, Merlin designed it most likely to secure it from beings a lot more powerful and bad assed than either Eb or LTW..

Which is why the simplest response is to slam the door on the well. Smallest area to defend. Let the invaders run nuts on the surface. and if they can break contain, the big bang.  But Harry, with the Wardens and Seniors, is not inside that perimeter. Alfred's main duty is preventing escape. Not protecting the warden. He has kept the doors closed without any warden at all, and will likely do so again.

You presume the Denarian knows what Nic is really planning. I do not think ANYONE, other than the actual Fallen, knows that. Casting Dierdre down to the underworld? Do you think she saw that coming? And that was his daughter

As things stand now, I think we do:  Harry stated as simple fact that -- with the island's defenses on-tap -- he could have shut down the entire Outsider-assault on the island, without allies or other assistance; and that was a massive assault with multiple entities involved (including one of the Walkers).

and we know that isn't simply a keep-out dome like the one Molly used on the island already? Could be an offshoot of that research - Harry does shields well,

We don't actually know what sorts of low/mid powered "mass" defenses the island has, or how much of it can simultaneously-target multiple opponents.  Human minds can only encompass a few at a time, so the island would need to target independently... and do it *fast,* because each and every warden there could likely launch a fatal or disabling attack within a split-second (because that's  the speed they need to be, at the level they're working -- if it takes you a whole second vs. a blampire, you're already dead and it's moved on to another target).

That being said... I'm pretty sure this occurred to O.G. Merlin, and he wouldn't have left such a gaping hole in the island defenses.

But do the defenses extend to Harry? Or just the cell block. And how many of the SC realized Harry had made the link. Even Rashid didn't know until Harry told him to check

and bad information can be worse than none. Luccio thought Ivy was on a level with the Summer and Winter Ladies. Harry pretty much says "More Mab or Titania". The wizards had her way underpowered. And missed 90% of Mavb's army, because it is at the gates. And Rashid never corrected that