Some misunderstandings here are the Arbiter position.
To be an arbiter, you must be an member of the accords. Which means either you are a representative of a supernatural nation or a individual freeholding lord like the Archive or Marcone. By definition, you are either a power, or backed by a power in the supernatural community.
In this context, the arbiter will enforce the conditions of the arbitration. By that I mean Keep the Peace while negotiating and/or enforcing the terms of a dual. In this they are backed up by the weight of public opinion (at least the public opinion of the other accorded powers) who have an interest in enforcing the rules. This is a dangerous commitment at times, which is why many of the lesser powers do not try to act as arbiters.
The arbiter is NOT signing up to enforce the agreements that are the result of an arbitrated negotiation. In the real world, if the US hosts peace talks between France and Vietnam, they are not signing up to force the two participants to do what they promised to do. The supernatural world is no different.
If Marcone is hosting the peace talks, then all he signed up to do is provide a suitable venue, protect the participants and severely punish/kill any that disrupt the peace talks (in some way contrary to the agreed procedures). That might be a commitment beyond his capability to deliver on, but the presumption is that --- as a free standing lord - he can deliver it. If he fails, his status as a free standing lord would be put at serious risk. Of course if he succeeds, then that also lends a lot to his reputation.
I more or less agree with your description of what Marcone's role is going to be, but I really dont think Arbiter is the correct term for it. Arbiter is the term used for what the Archive was during the Duel, which was providing the mechanism of the Duel while
Enforcing the terms of the Accords, but it was notably within her purview to Declare one party or the other the Winner or Looser or in Violation of the terms of the Duel (she declared it Void but did not have a witness to definitively call a winner). Marcone is acting as the
Host of the Talks, meaning he has to provide for the attendee's needs like food, lodging, etc and is nominally responsible for providing security, at least from Outside threats. But nobody expects him to be able to Enforce much, but since Mab is personally in attendance he shouldnt need to address that sort of internal squabbling, because they'd be insulting the Unseelie Accords before they'd be insulting the Host. At the end of the day it is always Mab that is Enforcing the Accords, that's the crux of how they operate; Out of everybody, the Council or the various Courts or the Erlking or anybody, it is Mab that is the Power and the Threat behind the Accords.
Would Marcone's active war with the Fomor invading his city compromise his neutrality?
Not in this context, though it's fuzzy on the specifics. I think it just doesnt really matter to this specifically since it's the
Fomor that have been repeatedly violating the Accords, so they are arguably at war (or at least at odds) with All the Accords members.
How does that conform with the White Council having it's own security? I mean it could be for when the White Council representatives are not being protected by Marcone, at whatever safe house they use while in Chicago, but that doesn't feel right to me. I think it's more likely that Marcone made an agreement that goes something like this; Marcone will provide external security for the venue while the individual parties are responsible for providing their own security while inside the venue.
I think those would more or less be separate issues. Marcone is acting as the Host, so per normal Guest-rights he is responsible for protecting his guests from outside threats, but those rules are less cut & dry when it's a matter of one guest being aggressive toward another (Erlking was well able to declare a friendly combat to the Death, for example). So it would be on Marcone if, say, a local threat were to hassle one of the Accorded Attendee's. But if a pair of underlings got into a brawl it would be a diplomatic matter between those two parties, with the slight getting aimed more at the Accords themselves (and Mab) more than Marcone.
In other words, Marcone is responsible for the venue, but Mab is still leading the Coalition.
Of course it's highly likely that the parties to the talks supposed to pledge to a non-aggression pact while taking part in the talks, but no one really trusts each other so everyone wants to have their own protection with them. Someone like Ivy would have to power to punish (obliterate) any transgressors of the non-aggression pact. In fact, I could see one or more parties involved; assuming there are more than two, demanding Ivy or some other neutral arbiter; with a similar scary level of destructive magic ability, be hand to insure everyone abides by the agreement.
Yup, that's how Carlos described it:
Everyone will be conducting themselves under guest-right, but they’ll all bring their own muscle too.” “Trust but verify,”
