Most other shapechange effects are merely spells or class features. They all last long enough to easily be used for prep, and can easily be activated, perhaps mutliple times, withing the course of a single combat. The druids limit is meaningless by 8th level, as yo ucan hold a form all day, and resuse it multiple times.

While making it a race-specific class could be flavorful, I don't see why it couldn't be an option to everyone. Sorcerers have powers stemming from their bloodline, the trueshaper could be said to originate this ability from their bloodline as well. Or it could just be an arcane abilty they master. The flavor of why they shapechange can be left up to the campaign.

I was going to go with full BaB, which gives it an edge over a normal eidolon due to hit dice. good fort saves, perhaps reflexes as well. Changing them based on base form would be kinda weird. An eidolon is locked to a single base form, and so tieing the saves to it makes sense. I would say change them, but the base forms allow yoou to get good will saves, and I don't picture this as a class that is mentally flexible, just physically flexible.

I think it is perfectly reasonable to assume that your base humanoid form serves as the biped base form, and adjust the physical stats for the other base forms relative to it. Not to imply you couldn't alter the looks of your biped form to correspond to what you want, but I picture your base race would be the "default". This does come around to the question of if they should be able to use it as a disguise. To disguise themselves as "not me" seems a given, but at what point, if ever, should they gain the ability to control the shape change precisely enough to disguise themselves as someone else? I'm thinking that if it is allowed, it should occur when a spellcaster would be getting alter self (I think 5th level, off the top of my head). It definitely fits into the flavor of being a shapeshifter, and I doubt it would unbalance the class.