I think you're right. Disagreement about the (potential) validity of Arguments from Authority is only a secondary outcome from what we consider Arguments from Authority to be.

I guess you're right.

I would not quite draw the line in the same place but it is perhaps best not to argue over the details.

Sounds reasonable enough.

I agree that this in invalid (and my intuition agrees - I physically flinch if I imagine myself writing that). At the very least it needs an additional premise.

I added a premise and reworded the conclusion to match the standard formulation of the statistical syllogism here, but the argument form remains invalid (although, like I said earlier, it has the potential for high inductive strength depending on the size of X).