Explanation for Pregnancy

At D's trial for sexual assault of V, V testifies that D raped her and that she became pregnant as a result. D seeks to prove V's prior consensual sexual activity with X on the theory that V became pregnant by X and then falsely accused D of rape to provide an alternate explanation for the pregnancy. Would the following proffered evidence be admissible?:

a. cross examination questions asking V whether she had engaged in sexual intercourse with X?
b. testimony by X that he had engaged in sexual intercourse with V?
c. DNA evidence that V's unborn child was not the child of D?

In some sexual assault cases, evidence of the victim's sexual activity with others is admissible.

Rule 412(b)(2)(A) provides that evidence of the victim's past sexual behavior with persons other than the accused is admissible on the issue of whether the accused was the source of semen or injury. This contemplates the situation where medical evidence offered by the prosecution shows that semen or injuries were found on the victim, and the defendant's response is that he was not their source.

The rule does not address the pregnancy situation presented by the problem. Are the situations similar so that the exception should apply?

If this evidence is not admissible by logical extension of Rule 412(b)(2)(A), should it be admitted under Rule 412(b)(1)? (Constitutionally required?) The accused may argue that to exclude this evidence would be to deny D the opportunity to present the core of his defense. Moreover, this evidence does not go to V's general sexual character; rather, it goes to her motive in accusing D of rape. Thus, it could be argued that it is independently relevant, like evidence of motive that is held admissible under Rule 404(b) in other types of cases as not implicating the propensity rule. Do you agree? What other facts would you want to know before deciding?