Diluting will mess with the Ph, so I am not sure how good of a route that would be. You would be introducing more possibilities for infection
You are probably right, but at this point I basically have nothing to lose. It appears it is going to be a ruined batch.
Try top cropping off of a different batch at high krausen. Ive gotten a few points to shave off of apparently finished ferments this way.
You can always try Beano or Amylase.
Not the best solution but worth a try before you dump it.
Both break down starch into simple sugars, they just work a little differently from each other.