What you are saying then, I think, is that the right to bear arms is not an absolute right. But you seem to prefer the arbitrary whims of some unidentified ‘guardian’ to a set policy. If any restrictions are to be made then I prefer that they be made at the state level and not the federal level. As the 10th Amendment provides.
Subject to Fourteenth Amendment protections, states are free to set their own standards with regard to how people are adjudicated mentally incompetent, and how the affairs of such persons are managed. It would seem that the legal standard for finding someone sufficiently mentally incompetent that they cannot be allowed to do things like make their purchasing decisions should be pretty similar to that required to disarm them on the basis of mental incompetence.