A few musings...  Do we know if Justine has any magical capability of her own, or is that even needed as long as you perform the proper ritual for summoning outsiders (like following a recipe), given that these are things that WANT to enter our world, whereas other beings might object to being summoned? Or could it be that she is a willing host for the walker, and the walker then temporarily granted her the magical spark, in the same scence that the fallen angels in one of the denarian coins can lend you powers temporarily, and they act through you?

for some reason i think at the time they met, the party, I think she offered her hand to Harry. If she were a practitioner of any power, he could have told them

Given Wizard secrecy, it's easy to see them losing "tech" that was held as individual trade secrets, but not losing public customs. 

Like them still unable to truly recreate Damascus steel? Loss of tarce elements in the raw material? techniques? We are not quite there even now

I point out that when the SC went to the island, Eb, Joe, and Mai arrived by boat as well. Recall that Jim also said places near nasty places in RL tend to be linked to nasty places in NeverNever. So what does DEMONREACH link to? Margaret knew more Ways than anyone - doesn't mean she knew them all. Might not know Rashid's. Or... wasn't there word he walked across the Moon a short distance for one? maybe that is why Harry can't do it? And Peabody traveled them with someone. NN creatures might be leery of attacking his companion, but not afraid of Harry. Cowl? a Walker?

Maybe Harry sees the trip as a way of prepping himself for whatever comes next - shift from Real World to island thinking. And it shouldn't take THAT long - while we don't know where the island is (there are no islands in southern Lake Michigan - that we know of) the lake is only about 45 miles across by Chicago. I'm a little surprised Thomas doesn't have a nicer boat, but maybe he figures like most Big Brothers, if I get something nice, my little brother is likely going to break it. (should have been able to find an old diesel engine and slap that in, give it some pep when needed)

If I remember correctly there is also a woman in one of the short stories in Side-Jobs, who is a fae and uses a credit card. With a microhip in it if I recall correctly (and as far as I understand it, the US is way behind Europe for that).  Don't have the book, so I cannot verify, but I recall Dresden realising that she could use it meant that she was supernatural and not a wizard/sorcerer.

We have plenty of chipped credit cards. Even the riff raff like me had one for years.

I will point out though that the head jailor is called a warden, keeping the criminals in.

Also - the Whamps call humans sheep - game wardens control hunting.... protect some wild animals (people)... keep an eye out for new strays (warlocks)?

Both warden for Demonreach and the WC can apply

When I say the Merlin has the votes he can do anything, he might get a vote to strip Eb of the Position of the Blackstaff, for refusing to take on Harry, and seek to award it to some favourite to take on Harry as a power up. Internal politics.

Now how much the Merlin knows about the Blackstaff and it origin is moot, as the Blackstaff seem to have generally passed on the Blackstaff to their chosen successor. Everyone out of the line of succession may know very little other than what it does. He may not know that it is an object of Winter, and that it may be ineffective against the Winter Knight (Eb didn’t draw upon it in his battle with Harry) 

I think more likely Eb was trying to keep it nonlethal, and might have been afraid the Staff would take it upon itself to kill Harry if an opening presented. Power-wise, the Knight is a step down from the Lady, who is  step down from the Queen, who is a LONG way from the Mother. The staff is from Mother Winter. She is not carrying a .22 short.... 

Harry does mention Maggie. He talks about money he's left aside for her and how he has barely enough left to run the castle for 18 months.

18 months is fine> He'll be a trophy husband by then, and the little woman can afford it.....

Could it be a "sufficiently advanced" machine, sure. Or it could be a mortal cellphone that was used as a plot device to show that Molly is becoming less mortal. Either could be true but I think the second is what was intended/does more to progress the story.

I think Harry sees it as her becoming less human. Less Murphyonic field. i don't think Bob and Butters could do anything about it. The field is there or it is not. Hers is dying.  Also, the Ordo Lebes seemed able to use modern things. Less power, less interference. If Elaine was as strong as Harry, would she have come up with plugging in her chain? Doubtful, she'd be in a cold water flat somewhere without electric

Here's a question - if Molly is pulling power from the Mantle to do her work, the same way non-magic Fix does some stuff - is her natural ability withering from non-use?

For Harry? Carry thorn manacles and a phone in a shielded bag. Put on manacles to make a call. To be safer, keep phone powered down when not in use. No incoming calls unless manacled and waiting.