Entrances from the Nevernever exist wherever Jim needs them to exist.
Like mirrors, that from Proven Guilty. From Harry's closet in the Knight's quarters, Cold Days. In the middle of the train station in Small Favor by both the Hobs and the Gruffs. The Gruffs apparently could use the Oak Cluster as a homing beacon to move directly to Harry's current location no matter where he is, Small Favor. The fetches are drawn to a beacon as well, Proven Guilty. And is how the Grey Council gets to Chicken Pizza, Changes.
Mab can not only get to Harry wherever, she can shut down travel better than the TSA, as she shows in Cold Days. I suspect this last is because she is related to Hecate, the Goddess of crossroads and because she controls the Outer Gates, which may enable dimensional travel in some way. But that is me being me.
Jim of course can create whatever he likes, depending upon the amount of repeated questions he is willing to endure at Conventions.But the Jewel is a sonic screwdriver. In the older longer Doctor Who serials the sonic screwdriver was discontinued as it was too easy to use it to get the
writers the Doctor out of a weekly plot hole. With the more modern faster paced revival this isn’t so much of an issue.
A journey through the NeverNever isn’t just a journey from A to B it’s a character or story development point, except for Changes where suddenly Harry acquired the Jewel and had to get from A to B rapidly without major character or plot development. Jim needed to speed up that part of the narrative.
Mab was able to shut down access from Winter to the Never Never and the mortal world to give Harry a head start. She didn’t shut down the Never Never to Chicago. She has control over her realm, no particular control over other parts of the Never Never. The are a lot of gods and deities associated with crossroads and doors.
Summoning depends upon whether the being summoned wants to answer that summons or not and whether that summons is strong enough to override that beings will. Mab wanted to go to Demonreach, going via the summoning circle on summons by the Warden means she didn’t have to fight the Demonreach defences to do it. She had an invitation. Still she took a risk, as Harry could have put her down then if he chose.
Use of a name gets attention, sometimes negative, which Harry is good at.