Can a prior FV case that was 12.45 into another plea be used for a continuous FV incident under 25.11? My gut instinct would tell me "no" but I can't find a definitive source to tell me otherwise.

I don't think so. 22.01(b)(2)(A) allows the enhancement for someone previously convicted. 22.01(f)(1) requires the defendant to be adjudged guilty, enter a plea of guilty or nolo contendere in order to receive deferred adjudication in order to be used to enhance. 12.45 requires the defendant to admit his guilt in order for the offense to be taken in consideration in the punishment of another offense. While the defendant does admit his guilt there is no plea of guilty or nolo and no adjudication of guilt. Hope I am wrong but I don't think the 12.45 will meet the statutory requirements to enhance. Might look at Lopez v. State 253 S.W.3d 680 where the court held that extraneous offenses considered under 12.45 do not constitute prior convictions available for impeachment under 609. Not quite the same issue but appears that the same logic would apply.

I 12.45 is a bar to prosecution. You don't need a conviction under continuous family violence to use the offense but 12.45 is an agreement that it can't be used to enhance because 12.45(c) says prosecution is barred for that offense. Because you used it in sentencing it would now be a double jeopardy situation to use the act under 25.11 the continuous family violence statute.
Brent would be correct if you were trying to enhance under the traditional family violence enhancement with prior conviction.