I'm still not sure, many talents that have multiple levels, the character can buy one or more of them, but for example if you buy the next Tough Hide up from the one you already have, it only provides the difference between the DB bonus of the two Tough Hide Talents.
You are correct, Imbued With Element is a racial ability for the Bentaxa, it is "like" Affinity of Element with the exception that all parts are doubled. So it would be useless for a Bentaxa to take Affinity of Element for the same element as their Imbued with Element, taking it for any of the other three elements would still make sense.
Elemental Defense provides the least of all three.
So breaking it down:
Elemental Defense <Element>: DB Bonus +10 vs. Element (from Bestiary)
Affinity of <Element>: DB and RR Bonus +10 vs. Element, Casting Bonus with Element +5 (from College of Magics)
Imbued with <Element>: As Affinity of Element, just doubled. (from Folkways)
Thus, if you have Elemental Defense (Fire) and Affinity of Fire, you only get a +10 DB bonus vs. Fire but you do benefit from the other parts of the Affinity talent.
Also, point of clarification: Core Law?