Time to time, well, more often than not, current mormons accuse former mormons of all kinds of less than honest behavior. From purposely misrepresenting mormon doctrine, to not being honest about experiences they have experienced to even not ever truly understanding what they were taught or told during their time in mormonism.
You are, in essence, trying to defend a logical fallacy, namely an appeal to authority. As such this entire line of reasoning is faulty and underdeveloped. You cannot defend a logical fallacy.
The authority one gains by being a former member of the LDS Church is ultimately non-existent, unless you are seeking to make a false and misleading authoritative stance. This attempt at false authority is dishonest in the sense that it is fictional, though the actual intention behind the act is ultimately unclear.
The fact that you must address, and have not, is that membership in a group does not instantly grant authority. In fact the appeal to instant authority is a terrible premise to rely on. If this is the stance that is being taken, it can only be because more legitimate stances are unavailable. This is why logical fallacies are used and defended, because legitimate argumentation is not available.
Historian Jan Shipps likely knows more about the LDS Church history than the vast majority of members, and non-members for that matter, and she is not, nor has she ever been, LDS. Fortunately for the LDS Church, and unfortunately for antagonistic LDS critics, she is very positive, overall, about the LDS Church.
So simply because someone is Jewish, Buddhist, Muslim, Hindu, Christian, Zoroastrian, etc... does not convey understanding of the nuances of that faith. In fact in many cases a claimed knowledge through simple membership becomes a hinderance and a stumbling block. When someone claims particular status, they are an ex-(insert position here) they are setting themselves up for a significant fall.
The recent case where a claimed Gospel Doctrine Teacher failed to understand a simple theological premise that would have been taught in Gospel Doctrine, only exemplifies the unstable nature of the claim to have taught the claimed class.
Your entire OP is based on the erroneous assumption that there is some basis for authority based on past membership. There is not.