The logic of the immaculate conception is not compelling, imo.
Nor is the hemeneutic compelling.
We, therefore, have a combination of assumptions being combined to create a doctrine. Since premised on assumptions, it cannot be binding. It certainly is not authoritative.
"We, therefore, have a combination of assumptions being combined to create a doctrine."
No, the facts of the case and hence the doctrine existed prior to the assumptions. The assumptions are merely explanations of the doctrine.
"It certainly is not authoritative."
Correct - for you! But as you are extra ecclesiam, you hold an a priori rejection of external authority anyway. Do you believe that anything is binding?