Whoa, whoa, whoa- first thing, the Robe of Eyes doesn't grant True Seeing. At all. It lets you see invisible and Ethereal creatures, yes. But it doesn't let you see through illusions. All of its benefits are clearly spelled out in the item's description. True Seeing is required to make the robe, yes- but by no means does that mean the robe grants True Seeing to anyone that wears it. Here's the relevant text in case you're confused:
Robe of Eyes: This valuable garment appears to be a normal robe until it is put on. Its wearer is able to see in all directions at the same moment due to scores of visible, magical eyelike patterns that adorn the robe. She also gains 120-foot darkvision.
The robe of eyes sees all forms of invisible or ethereal things within 120 feet.
The wearer of a robe of eyes gains a +10 competence bonus on Search checks and Spot checks. She retains her Dexterity bonus to AC even when flat-footed, and she can’t be flanked. However, she is not able to avert her eyes or close her eyes when confronted by a creature with a gaze attack.
A light or continual flame spell cast directly on a robe of eyes causes it to be blinded for 1d3 minutes. A daylight spell blinds it for 2d4 minutes.
Moderate divination; CL 11th; Craft Wondrous Item, true seeing; Price 120,000 gp;Weight 1 lb.
As you can see, there's no mention of seeing through illusions or transmutations or the like.
That being said, if for some reason someone CAN see through illusions and such (such as an actual casting of True Seeing), there's lots of things you can do in this situaiton. I just want to say that I hate the idea of letting the PCs find out she's an evil undead dragon and then just strong-arming them into doing what she says anyway- it completely ruins the entire plot twist, and no matter what anyone says, you should avoid them finding out her true nature at any cost. (I ran the Age of Worms last year, so I know what it's like.) Here's what you do- first, the thing to remember is that Lashonna has access to some powerful magic- her own spells aside, she's got a Ring of Three Wishes in her horde, as explained in Dawn of a New Age. This is a bit outside of the realm of the campaign as written, but here's your excuse: The Lashonna the PCs meet isn't actually Lashonna. Not directly, anyway. It's actually a Simulacrum- the result of a powerful Wish, of course, because a Simulacrum spell by itself wouldn't work in this situation (can an undead dragon even make a Simulacrum?). It's a Wish-created Simulacrum that looks, sounds, and registers as Lashonna's elven form, while it's completely and totally controlled by Lashonna. Seeing as how it's a creature that looks exactly like the Elven Lashonna, True Seeing would show it as it is- a creature that looks exactly like Lashonna. And since it's controlled by her and her alone, it's Lashonna for all plot purposes.
It's a bit meta-gamey, but hey- your job as the DM is to create an environment in which the PCs have a good, fun, experience, and the better the plot, the better the experience. It's well within her power to do this, and really, there's no reason she shouldn't.
But, of course, the most important factor is that a Robe of Eyes doesn't grant True Seeing.
