Yes, he could have illegally married her, if his marriage to Kezia was in fact legal (it might have been some tribal ceremony that was not legally recognized).
If he was really a bigamist, then his marriage to SAD could be challanged and legally declared null and void. However, until such declaration, the law would presume the validity of his marriage to SAD.
At any rate, I don't see why this matters. Why are you so interested in it?
Barack Obama was never involved in the divorce that you are referring to.
Do you Obama Jr. or Sr.? Of course Obama Jr. was not involved, but Obama Sr. was, as he was the defendant!