(NOTE: Dragon power comments are based on CRs for a Great Wyrm of that species.)
The problem, in my mind, is making a logical society out of this. Think about it for a moment -- Chaotic Evil creatures don't plan much for the future and don't have any compassion except for those they care about.
The only Lawful Evil dragons (normally, exceptions apply) are the Blue and Green Dragons -- middle of the road in power. The Black and White dragons are weaker, not as much of a threat. But the dominant power -- the Reds -- are Chaotic Evil. The great red that organized this aside, they're going to have a real tough time ruling.
In my opinion, this would lead to a fragmented, near-feudal system with each dragon fiercely maintaining his own borders. Conflicts become more and more common as time passes; dragons forget the potential of the lesser races for dragonslaying and focus on one another as the greatest threats to power.
Territoriality (sp?) rears it's ugly head. Dragons might actually encourage groups of monsters (and the PC races are included in that group) to attack their neighbors. Near continuous fighting along the borders of many provinces would eventually break out.
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On the surviving metallic side -- if you choose to have any survive -- the chaotics wouldn't fit in well with the new society. The Brass and Copper dragons are the weakest and second weakest of the metallic dragons. They're also the only chaotic breeds. Bronze, Silver, and Gold (in that order of power) would easily eliminate them if they became a part of the new, draconic society that their lawful tendencies would support as the ruling government.
Groups of rebels -- despite the fact that it was my original idea -- wouldn't have much of a chance at survival.
Groups of "lesser race" rebels would have a much better shot. I could easily see a wily old Blue (my favorite dragon, actually, so pardon me if I seem prejudiced...

) sponsoring a group of hunters that would go on missions to de-stabilize his enemies' lands. Worst comes to worst, the dragon denies all knowledge or has them assassinated. With a draconic patron in their early missions -- helping them to avoid magical notice, giving them a home base, perhaps even doing some illegal training in magic -- a group of adventurers becomes much more likely. Even the dragon's inherent capability for Hallucinatory Terrain, Veil, and Mirage Arcana would be very useful in thwarting scrying magics.
All of the mission ideas you've already presented make a lot of sense from this standpoint -- smuggling magic to their draconic lord, gathering weapons and fighting the half-dragon ruler of a neighboring city (or country) that offended their lord, and then perhaps eventually joining with an underdark band to overthrow their own lord when they feel strong enough.
Just a few more thoughts.
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Final idea, and this one is probably the most radical.
If you cut off the divine and infernal planes -- which makes a great deal of sense -- a more elemental form of magic might make a lot of sense. Introducing something like Sovereign Stone magic, which is based on the four elements, would make for a very interesting variant world. Allow druids to maintain their old school casting if you like, since their magic never dealt (much) with extra planar power. The other option is to make all druids fire/earth casters, which gives them access to druid-like spells.
You could always introduce PrCs that deal with the old magic that is now lost to all but dragons. Note that this makes dragons somewhat nastier, as SS magic is inherently weaker but more prevalent. Or, you could make it so that dragons severed themselves from old school magic when they sealed off the outer planes as well.
SS magic works on the basis that the gods aren't around or don't interfere -- so the human gods being dead works quite well. Earth magic takes care of healing.
This change -- to SS magic, which is a WONDERFUL system -- would make the world truly alien and different to most D&D players. And ANY spell from D&D can be easily and quickly remade into a SS spell -- so "lost" magic is easily incorporated into the new system.