How compelling are the arguments for the monoclonality of tumor cell populations, and what logic and observations undermine the conclusion of monoclonality?

First determining if the cells in the tumor have common and the unique genetic and biochemical marker and if they are present, that would clearly mean that they all were created from a initially mutated cell and formed the marker and these are considered as monoclonal. If that is not the case and cells do not have a common origin, these are called as polyclonal.

Yes yes yes very good