Best wishes on a successful surgery, and a speedy recovery.
As far as the possible use of a water pik, I just wanted to repost this tip that gmcraft posted to me, regarding the tongue cleaner attachment. I found it very useful for my thick mucus. (I had developed it twice, from my lingual tonsillectomy, and then from my radiation treatments).

"For the mucous, you can clear it with a Waterpik on the lowest setting, using the tongue scraper attachment with warm water with a bit of magic mouthwash thrown in. I was going to suggest it, but seeing that you only have 3 days left, I wasn’t sure if you wanted to invest in one. If you use it a few times a day, it clears the mouth of mucous. That, I believe, will help with the mucous at the back of the mouth too."