Make sure there are no punch points locking the adapt from twisting off with filter. Use a propane torch or hair dryer. Get the glue hot so the pvc tool starts to back it out when hot enough, still cooking it off. Pinch points are removed by using a larger drill and just V cut the indent away and stop (if applies).
The threads will be 3/4 -16. Which is a fine thread 3/4. There probably isn't enough threads to get two nuts on it. If you can't find any, let me know and I'll machine a couple down for you, or make you a tool.