Anyway, as a lover of all things Far Realmsian, I have to say I love this PrC. If my abberation-obsessed summoner character hadn't already taken levels in alienist, I'd be petitioning my DM to allow this one on the table...
...with one exception. I think adding a template that creates the equivalent of a +4 level adjustment might cause some balance issues, though I haven't done any serious number-crunching. Since he'd have to be at least 5th level to take the PrC, he'd be at least 15th level, but he's suddenly being bumped up to the equivalent of a 19th level character. A lot of PrCs (and a monk's 20th level ability) result in changing the character's type - usually to outsider or elemental - but to my knowledge a type change doesn't result in any sort of level adjustment.
The only exception that I can think of is the Warforge Dreadnaught PrC, which changes the warforged into a full construct - a creature type that comes with significant ability-related advantages - but with that class it slowly builds on the warforged's initial construct-like abilities, level by level, until the last level of the PrC results in the character being completely a construct.
What you might want to do is have the PrC give the character one or two abilities of the Lesser Worm that Walks template each level, until at 10th level it has all the abilities of the template. That would probably take some serious retooling of the class, but otherwise you might just want to give the character the pseudonatural template at 10th level. That's only a +1CR adjustment.
Anyway, despite that criticism, as I said, I love it. I'm also a big fan of your Lesser Worm that Walks template, even if I don't think it should be a class ability. Keep up the good work!