I just wanted to state that I don't see any issue with the class having access to multiple elements, since it's a partial-casting, partial-BAB class. You're entering this class in one of three ways:

1) Level four, with three levels in Duskblade, in which case strong class features would be the only thing preventing this class from being a blatant downgrade.

2) Level seven, with six levels in Wizard/Sorcerer/whatever, same as 1.

3) Level five, as a multiclassed caster/full BAB class. This is mechanically bad, unless you take levels in one of a number of full-Casting/full-BAB prestige classes - many of which have good class features to boot. See 1.

So really, don't obsess too much about this class being overpowered. In the worst case, it's no more powerful than the character would have already been; in the best case, you're giving a boost to a mechanically inferior playstyle.

I'll hold off on any further judgment until after you do the rewrite. One question - what balance point/tier/what-have-you are you aiming for?