Are they accorded? If so wouldn't Marcone be within his rights to ask for compensation or declare war, etc?
I believe that bit of Accords politics is addressed in "Even Hand," but I have only heard that secondhand.
1) Be something that isn't all ready a signatory to the Accords
I think this is a good discouragement of the plan. Since warlocks are technically under the White Council's authority *per the Unseelie Accords* (for what that is worth), it may just be that they can't file to be a separate faction (under the Unseelie Accords), whether or not they have the juice and the will to go rogue. But that's our interpretation of the setting and existing canon: that it can't/shouldn't be done.
That said: if you definitely want to go forward with making this a plot element, then I think the bit about having three sponsors is a good start.
Also, you may want to sketch out the narrative role this is serving:
Is this background or scenery?
Is it part of a major Threat's master plan? Or is it something which helps to establish that Threat?
Is this maneuver something the players have a chance (or motivation) to prevent or undo?
Is this to complicate the players' lives vis-a-vis providing non-Council spellcasters who can't be summarily executed?
Is this going to be a faction which the players could join, or which would want to recruit?
When all is said and done, you could... ::looks around nervously:: ummm, you could simply say they are already a Signatory, or that enough legal tomfoolery had occurred in the last few centuries to establish this as a sketchy but legal Accords maneuver. Something the White Council was hoping to quietly squash, but failed to stop.