Drain pipe clogged in tough spot
I think at this point, you should call in a plumber with a snake &amp; camera to determine the problem with the drain pipe. Instead of cutting, flushing and making a big mess a plumber with a camera on his snake could quickly determine the problem and the fix needed. If you call in a professional tell him/her the size and type of the piping you are dealing with.
my 2 cents