I am worried about the fact that these shadow bitcoins can enter the market at the most inopportune moment and in one way or another affect the movement of the bitcoin price.
Why do you think that those who own those "shadow" bitcoins (as you called them) would mindlessly just dump them ? After all, they are interested in profit, so why would they behave any different than others?
To answer you question, no, I do not worry about such scenario. Bitcoin survived so much already and at this moment stuff like that can't hurt him in the long run, and that's what's most important.