Okay, I think I've located the source of our disagreement (and it isn't about validity at all). The term "probabilistic argument" is ambiguous.

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 have been using the term to refer to arguments that rely on inductive inference to move from the premises to the conclusion (in other words, the evidential link between premises and conclusion is less than perfect or the inductive probability is less than 1, since 0 and 1 are not probabilities). Alternatively, you seem to be using the term to mean an argument made up of statements that contain probabilities.

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

Argument Two

• X percent of F is G.
• H is an F.
• Probably, H is a G.

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.