The Immortal Flames are Ul'dah military while the Brass Blade and Stone Torches are more akin to the police force (though the obviously more personal police force of certain members). Even if the Brass Blades/Stone Torches weren't there doesn't change that Ul'dah was there.
And if the Immortal Flames could have removed them would have done so by now. It's not so easy as "taking over the garrison" the IF aren't attached to the BB they are two separate entities run by separate people. If Raubahn tried to get rid of the Brass Blades it would have only ended with war within Ul'dah.
The Sultana's plan was the best and well...we know what happened there.
Edit: After re-reading this I didn't flesh out my thoughts enough.
The Brass Blades are an army but a personal army for Teledji/Lolorito. So using might against them isn't the solution. It'd be a better bet to try to convince Teledji/Lolorito to disband them and we know that they wouldn't. So the only solution would be to remove the two from the Syndicate. Which would only lead to public outcry as they did nothing wrong other then have the Brass Blades under their control. Raubahn then should also be removed from the Syndicate for being the Leader of the Immortal Flames.
Nanamo's plan was the best option because if Raubahn was the leader of Ul'dah he could then force the Brass Blades/Stone Torches to be disbanded or at the very least weakened.