The individuals with the mutation contribute to the herd just like all of the other individuals so you can’t say they are using herd resources. They are equal members of the herd.
Later on when combined with another mutation the combination does provide an advantage and natural selection kicks in and that combination will spread through the population. The mutation is not going to create some new creature that predates on the population without the mutation. Over time, i.e. generations, the previous version will die out naturally as it is less successful than the new.
You keep thinking in terms of individuals and the theory is based on populations.
Ironically, you are right to some extent. It is a done deal, and the occasional discussions about the details are good, but the basic framework is a given. Perhaps that is why many have moved on to thinking about how other subjects like creation care, race relations, technology in health care etc. are now more prominent in the science and faith discussion