All I drink now is seltzer water, sometimes with concentrated (unsweetened)cranberry juice and it didn't make a difference. Still a better choice the carbonated beverages with sugar.

Mouth sores are ulcers typically, they could also be radiation burns and even thrush. They can be caused by chemotherapy alone in some instances.

The thick stringly mucous is more of a discharge from damaged salivary glands. In my case after a mouth or so post Tx it just stopped. But it took a good 18 months (with IMRT) to regain about 80-90% salivary function.

I found that sleeping elevated helped a bit and having a bottle of water constantly by my side was a must. It will pass, but it is a miserable time. I had a lot of sleepless nights during that phase.