I love this module. It's good 'ole fashioned D&D. Goblins, Hydras, and Dragons (oh my!)
I ran it for a good while and my players loved it.
I'm not that familiar with the Binder class, so I will only make general suggestions for this battle.
You seem to have a good handle on what you want to do to prepare. This battle is one of endurance, so make sure to not deplete all your resources (unless you're about to die, or you are sure you're at the final battle)
Any sort of long duration effects will be good, such as any sort of fogs, or poisonous clouds.
Along with blockades, having all the main routes in the city littered with caltrops would be a good strategy while you have any available archers in the city rain death from the rooftops. If mass quantities of caltrops are not available, I'm sure you could substitute broken glass, marbles, etc.
For the dragons, unless your party's ranged attacks are reliably hitting, have a dispel of sorts ready. Not killing the green dragon will make this encounter much more difficult. If you are able to somehow reduce their DEX to 0 (via ability damage, penalty, exhaustion effects), that will let you coup-de grace them. (Almost all dragons have a DEX of 10)
Be ready for your party to be called upon to "handle" the dragons while the rest of the forces deal with the regular army. Make preparations to deal with any "property damage" caused by said dragons.
Edit: I second ImmortalSoul's idea of using Haboob(from Sandstorm). I'd actually use it over Vortex of Teeth(SpC).
Haboob is 3rd level, duration is
minutes/lvl, deals comparable damage, and obscures vision in a 20 foot radius cylinder.
Vortex of Teeth is 4th level, duration is
rounds/lvl, deals 13.5 average damage/round, however the radius is twice as large.
Good Luck!