I'd be happy to provide feedback as your characters, who are Epic-tier experienced and smart, would understand them.
Gromley's plan:
Basically storing you all in a special space while I head in on my own acting as a diplomatic envoy. Then, when there are the fewest about of foes near our target I 'uncork' you all and we attack.
Highest risk, highest reward. You don't even know if you can GET those two extradimensional spaces inside each other. It's never been done. It could kill the people inside instantly unless they were in stasis or the like.
Agon:
1) Stealth: Stick to narrow alleys, avoid patrols, get as close as possible by being sneaky.
This is the most straightforward means for an experienced crew like yours. In metagame terms, think of this as Stealth+Athletics+Acrobatics. Those who can't sneak can climb or balance. Everyone in the party would roll their favorite of those skills. It's also slowest, though nobody--repeat, nobody--put a timetable on this for you.
Agon:
2) Disguise: We have totally legitimate business in the City of Brass. Definitely not a squad of assassins armed with an artifact.
This is a combination of magic, Bluff and Intimidate. This, alone, is a weaker option than Sneaking.
quote:
3) Magic: We're epic level badasses. Use flight, invisibility, extraplanar spaces, and sheer balls to go straight at the Ring of Fire.
Reminder that some of the efreets and all of their commanders are also epic-level badasses. Plus, your best chance at getting at your target is being SUBTLE. Fighting your way through would go badly for you. This is the least logical option as your characters would discuss it.
There is one more option you haven't mentioned:
COMBO of Sneaking + Disguise
This is combination of use of magic (e.g. Sonliin turning himself into a dragon), Bluffing (see Gromley + Zog) and Intimidate (Darratheir) along with a bit of knowledge & perception (Agon + Martha + Harald).
In summary, if you ask me what your characters might come to, I'd say they probably want to test Gromley's approach first in some non-risky way and, if it fails, go with the combo approach rather than something mutually exclusive.