I absolutely need Regis to meet pre-FFXV!Ardyn because it would radically change his opinion of the man and make him reconsider his commitment to the Astrals even more.

This would completely crush him. Regis would have no idea what to do if he found out the complete truth about Ardyn. He hasn't quite had the "talk" with others regarding Ardyn yet, though they have skirt around the issue before.