Not sure i have definitive answers to your question, but I go to Bang Phra every month or two and have often seen SB Pelican there. The problem with Bang Phra - as you point out - is that it's hard to tell what has either come from the breeding center, possibly escaped from the nearby zoo, etc. and so on, or just arrived naturally. I have also seen Lesser Adjutant there and Darter and Black-headed Ibis. With the Pelicans and the Darters I think it's possible they arrived from elsewhere. I have had Darter in Bangsaen, Chonburi and Pelican flyovers. I also had both on a reservoir I lived on in Surin, way up in Isaan. The Dratres are apparently making a comeback largely due to good protection in the Tone La Sap in Cambodia. The Pelicans I am not so sure. But i am pretty sure I saw a flyover from my car on the way back from Kanchanaburi to Bangkok last week.
On another note, I recently tried to enter the breeding center. They used to let you walk around. Not anymore. My wife and I were told to leave immediately and I was told to put my camera away.