Positions (1) (2) (3) are nearly axiomatically true -- if we are to generalize at all, and we must, we need to look for positions and people to generalize from. The very purpose of a professional organization is to aid us in doing this. Members of these organizations use it as a mental model to generalize "my professional community" and "the leaders of my professional community".
If you look at her resume, she is the former head of this and about 4 other high-prestige psychologists professional organizations, and a high ranking member of nearly every organization she is eligible to be a part of as a psychologist. She is the very definition as "well-respected representative of her field". She is a fine target for generalization. What you have not brought to the field are conflicting reports from other psychologists contradicting her or suggesting she is not an appropriate representative. Jimmy Carter and John McAfee are readily directly contradicted -- if you ask me to provide sources for numerous professionals in the field suggesting they are not the standard I can do so easily and so can you (though Jimmy Carter is a weird selection -- he is not a professional, and both are telling, since neither of them work in their field anymore)
Well, no; even we accept the arguments that (1) professional organizations of psychologists are a good proxy for generalizations about psychologists, and (2) the head of an organization is a good proxy for the organization, and (3) that each of those relations are strong enough that treating the relationship as transitive such that the head of a professional organization of psychologists is a good proxy for psychologists in general, she still wouldn't be the first place we would look to represent psychologists as a whole.
Because she's a former head of such an organization, not the current head. Just as we wouldn't look to Jimmy Carter as our first choice to represent what Americans as a whole believe (or John McAfee as our first choice to understand what the business organization now known as "Intel Security Group" believes), we wouldn't look to the former head of an organization as the first choice for generalization about what the group the organization is supposed to represent believes.