I like some of the mechanics in this class, but I'm not personally a fan of the flavor, I think it would be served a lot better as a base class with a more subtle seduction/assassin tone and then the tentacle/futa angle as a PrC.
Someone should redo the class like this.
Actually I disagree, but only because of the flavor. It's doable though.
It's not too hard to gestalt two classes together and then start knocking pieces out that make them unbalanced.
In this case, if you want a seduction/assassin martial adept:
First, just as an example, gestalt Beguiler and Swordsage. Something from book of erotic fantasies may actually work better than Beguiler, but if I remember right I didn't like any of the base classes in there (I'm doing this without books).
Now, eliminate all the disciplines but Shadow Hand and perhaps Setting Sun (because you know this class likes to switch positions...) and most of the illusion school spells from the beguiler's list that aren't subtle (I think they get phantom battle or whatever, needless to say I don't feel it fits..).
Also rip out all the straight combat abilities of the swordsage, like the discipline focuses. Eliminate armored mage from the beguiler and a lot of the best parts of cloaked casting (although, for the record, in order for this class to make sense as a 20th level class you want some of those). Keep the free silent and still spell. Introduce some sneak attack progression.
So far you have something that's pretty seductive and pretty effective as an assassin IMO. Personally though, I think there's something wrong with this concept as a
martial adept. The overwhelming fact about the ToB classes is that they are out to hurt people in good old fashioned combats. The Swordsage may hide from you, but they won't lie to you. A Crusader may have Diplomacy, but it's not so they can seduce you it's so they can negotiate your surrender after they've beaten you into a miserable weeping mess. A Warblade is literally someone who wants to beat things up and get lots of props for it.
"Seduction/assassin tone" doesn't actually work for ToB. They don't seduce, they destroy. They may assassinate, but they're not going to do it with poisons or daggers in bedrooms. Ballrooms and banquets and back chambers are not the place for
martial adepts, which is why I said I disagree. All of the classes in that book are unquestionably violent and straightforward about it (Again, disappearing into the shadows
once combat starts is not stealthy, so the Swordsage still is pretty directly violent if indirect in their assault).
It's one of my favorite things about ToB, even the 3/4ths BAB guy is looking for a violent solution. Most of them don't have Profession or Perform as class skills because just like the scene in
300 "What is your profession?" these people are violent melee killers first and foremost, they don't have another trade.
Finally, seduction/assassin types aren't actually D&D's favorite. The system is designed for teams, and such characters are frequently lone operatives. They can still work and often do, but it's usually best done via solo games or at least solo sidequests (I like going Assassin just to fulfill the special requirement aka get a cool clandestine assassination mission from my DM).
In a way, as tasteless and abhorrent as it is to me, this "Echinnobbi" is as sexual as something ToB-based could get. Do you think combat "maneuvers and stances" that manipulate a tactical system and action economy designed for battelfields can really fit in even the largest of bedrooms? If you executed a ToB maneuver on someone in a sexual way, it would most certainly be some form of rape. Sadly, someone felt like going with that angle. That kind of sex is pretty unrealistic though (Can you see two soldiers meeting each other on the battlefield and then endeavoring to bend each other over as a means of winning the combat?) and really doesn't have a place in any but the oddest of tastes.