I too agree with you that Mary is selfish in the belief that she can somehow redirect Edmund's future to suit her personal preferences.

But is part of her desire to change his profession driven by a confidence that with her, and her family's, societal connections that Edmund would be highly likely to succeed in either the military or politics? She may be placing a high value on the Crawford alliance, and has therefore confidenced herself that Edmund could be propelled rapidly in either profession. Whether that is true or not, is subject for conjecture.