Stick to your guns.  Your first obligation is to your daughter, and you have every right to simply follow the parenting plan, which I assume is in a court order or divorce decree, or included by reference in something signed by a judge.  She has no threats that she can make good on, because the court will back you up.

That very limited visitation tells me that your ex-wife did something pretty bad to your daughter, and I'd work from the assumption that limiting visitation to 2 hours at a time is something that might be really necessary for your daughter's protection.

If the parenting plan doesn't mandate visits with mom on Halloween, then her threats don't mean atnything.  Your past Halloween visits were courtesies, but not ordered by the court.  Do what you need to do on Halloween.  But don't eat too much candy!