I wouldn't hesitate to make this paranoid dragon's lair an underwater one, in line with draconic paranoia. In fact, it really should be somewhere in the dephs of the Dragonreach or the Thunder Sea to have the best effect. Syberis Observatories require a careful balance of energies between Khyber, Syberis, and Eberron (not to mention sacred geometry and arcane geography), and such locations are not always convieniently tailored to the random scholar's specifications. In my mind, the lair will be a singular exposed shaft that sinks from the seabed all the way down to the very dephs of Khyber, with a chamber about 1/3 of the way down that is suitable for an Observatory. Various forces and pressures keep things at the bottom from reaching up the shaft up into the rest of the world proper. Most of them.
Usually.
Of course, the problem with this is that with an underground chamber that only has light comming from a single shaft deep down (which remains unusually bright, due to a few alterations required for the Orrery) is the limited view of the sky. That isn't ideal, to put it lightly, for any Observatory. That's why there are specially bred species of plants, fish, and mobile coral that move and disperse in response to commands issued in the central platform of the Observatory/Orrery to display information recieved via sympathetic harmony with the three layers of Eberronian astrology. It should be able to be used for any number of effects both magical and mundance: most should die with your dragon, and will require extensive study and test (with results both disasterous and hilarious) to discover/comprehend/unlock. It's a McGuffin like the Web of Motes is, except it's tied to a location. Don't be afraid to incorperate a Mirror of Power for
MostinNameles' edification.
As for treasure, Gareth has lately been moaning, as he is prone to, about the loss of his armor. Let's make him happy by dropping in a set of nice Dhakaan armor. Say... a minor-artifact +5 (insert-armor-type-of-choice) major acid resistance with immunity to all beholder-kin eye-ray effects. The immunity to beholderkin eye powers also includes antimagic zones... which means *swift fly*-*charge*-*splorch*

Naturally, since it -is- a Dhakaan artifact, with dozens of blinded eyes (various methods) depicted all over, this results in many excuses for your patented Shilsen setups.
We've already discussed Nameless, so let's move on to the local horny sociopathic druids. Since you've limited the access to non-core spells, I recommend expanding their potential repitoire. Make it interesting by making it a plant from Lamannia, and plants grown from it form new and fantastical shapes that can inform them of new spell possibilities, particularly when you use
Speak with Plants or
Commune with Nature on them. Higher level spells/insights/secrets are available by using higher level nature communication spells on them. Since they're extra-planar plants, normal physics and biology assumptions do not apply to them. Stone plants, plants with large spheres of water as fruit, and flaming flowers are all viable choices, especially if growing them results in different kinds of plants each time, no matter what the parent strains are. Scrolls, potions, and oils grown from them can be great sources of weirdness too. Give them better stuff if they take the time to take care of them, with spells or otherwise. They should have viceral expressions, reactions, and psudo-personalities not normally present in normal Eberronian plants: try to make Luna endearing on some of the nastier agressive ones that doesn't like Nameless or Gareth.
Six can benefit from more support of his new line of Martial powers, so maybe it'll be a good idea to put in some more forms of Warforged 'fungus' to improve recharges and/or grant more powers/stances. Another shapeshifting weapon would be great in line with Six's adaptable nature: I recommend a spiked chain (it is his signature item) that can change material composition (+1 bonus), and have some equiviant bonus to the Sword of the Planes from the SRD. I remeber the Equalizer from Baldur's Gate 2, and it is a glorious, if somewhat underpowred once you got it. Binding it to Six so it teleports back to his hands every time he drops it is also great. Don't forget, the Giants made the Warforged against an incursion against extraplanar threats, and some of their developments should be effective against other foreign intrutions.