Whenever I add things to the TMX, I always give the mixture a quick stir with the spatula to briefly incorporate the new ingredients.
In regards to your mushrooms, how big were the pieces?
I do find that light ingredients like herbs do have a tendency to float, so I wouldn't worry too much about that
It sounds like it might be something to do with the temperatures if you say your mornay wasn't cooked either, but I'm afraid I've never had those sort of problems. Hopefully someone else will be able to help you more