Texas Family Code has clear alternatives for when there is difficulty for child exchanges. Call your atty and ask to file a motion to do three things: 1) clarify possession when child is in preschool; 2) enforce a public drop-off/pick-up location with times (it may end up being you all have to drive to an out of the way location, but there will be a witness to the exchange); and 3) enter documentation of the mother's difficult behaviors.
To further #2, Even a Burger King or some such location is acceptable. Go in and purchase something showing the date and time on the receipt. It's a paper trail to show who is being compliant and who is not. It is a HUGE PITA, but it will be something verifiable.
You have DOCUMENT, DOCUMENT, DOCUMENT. I cannot stress enough how important it is that these actions be documented. Hearsay doesn't count for anything in a court hearing. And yes, you do have to pick your battles, but when it comes to following the court order, everything is a battle with a non-compliant parent. When you're dealing with that kind of parent, you have to show you tried everything you could to force compliance, up to and including court time.