I honestly don't know about draining off the fluid. You would have a much higher chance of success than most people because of your experience, but at the same time, bettas are so small. The procedure would have to be pretty precise to avoid puncturing an organ and the trauma might be too much for him.
I did see what looked like stringy poo coming from him. Does he still have that? Have you seen any evidence of white parasites coming from him? Otherwise . . . one thing I'm worried about is the possibility of tumors or a tumor-like growth. These can happen in the abdomen and while a fish can live comfortable for a time with them, the fish does eventually succumb. In fact, one of my girls just passed two days ago from a tumor that was in her abdomen.
For now, I would advise continuing with the General Cure for another few days as well as continuing with 3 tsps of epsom salt per gallon. If you have any Indian Almond Leaf or naturally dried oak leaves, you can use those for beneficial tannins.