Have you tried the Grapefruit-Seed-Extract internally? I don't mean the mouthwash but the actual extract diluted in water? I definitely helps me. Just be sure and take it at a different time from the probiotics.
You said you have post nasal drip. I do too and the best thing I've found for that is adding a small amount, 1/2 - 1 teaspoon, to a cup of saline rinse (made with 1/2 teaspoon salt, pinch of baking soda). There is controversy over whether it is safe to use H2O2 (Hydrogen-Peroxid) internally so you may want to research this and decide for yourself. Also, I have read on other forums here that congested and draining sinuses are caused by a congested digestive system. Don't know how you feel about going "alternative" (ie cleansing, etc) :) When you think about the fact that the digestive system is connected on one end by the "bottom" and on the other by the tongue....Have you read some posts below? In particular Mr Cooties? I did my first Liver Flush Sunday night. Was amazed at what came out!
In any case, there is evidently no easy fix for us. I'm determined to leave no stone unturned till I find the answer!