Mixing vegetables in a blender (e.g. for soup) - does the percentage of fiber decline?
You seem to be mixing up three things in your question: the total amount of fiber, the degree of mechanical change to the fiber, and the health effects of eating fiber which has been mechanically changed. 

For the first, the answer is clear cut: there is no change at all in the total amount of fiber. Blending is not a chemical reaction, it won't turn inulin molecules into anything else any more than grinding lead will turn it into gold. 

For the second, it is unlikely that there will be a single answer. First you have to define exactly what kind of mechanical change you are interested in, and for any of these changes, to live with the fact that all accompanying variables (what blender you have, how long you turn it on, what are the other ingredients in the recipe...) will change the answer quite a bit. And you yourself found a specialist saying that people don't seem to have studied that. 

The third is simply off topic here. Any answer based on the health effects of mushrooms, either telling you something about them directly, or choosing how to answer the amount-related second question ("there are many types of mechanical change, but only type X is interesting from a health point of view, so we will tell you the amount for type X") is out of scope and will get deleted.