Ok, I was AFB, so I couldn't say much. Let's see now:
First, a class usually ignored because for normal games its usefulness is generally low: Cabinet Trickster, from Races of Eberron. 6 skill points per level, Change Shape and Read Thoughts at will with a DC based on charisma and class level, some small bonuses to bluff, intimidate and sense motive and a pair of feats (Racial Emulation can be fun for a spy). And, well, some tricks which can be useful.
That said, in the higher levels this loses utility due to nonscaling DCs
For the levels before that, try incarnate. They are awesome for any skill based character, and gestalting it with a rogue means you don't have to worry about losing skill points or attack bonus. 2 levels of incarnate gives you 3 melds, 2 essentia and a chakra bind. Silvertongue Mask with 2 essentia gives +6 insight to bluff and diplomacy. And you can chose your melds everymorning, and most are utility based, so that's a win.
Personally, and following your build, I'd go:
Changeling half-fey useelie fey rogue X // Marshal 1 / Incarnate 3 / Cabinet Trickster 5 / Chameleon X
Feats: Extra Essentia, Shape soulmeld (Blink shirt) [At will teleport is always good for an infiltrator], Force of Personality, Skill focus (Diplomacy), Persona Inmersion, Racial Emulation, Able Learner, and anything else you want.