u/No_Cricket4772

He instantly recognises that you are a vampire, but he doesn't bat an eye at Alva walking the streets, charming every man she can find. All the events of Laid to Rest could have been completely prevented if he was like 5% more vigilant.

My personal theory is that Movarth and therefore Alva are from a strain of vampirism not represented in-game. The Cyrodiilic strain that hides someone's vampirism completely. So while your case is clear to him, Alva isn't as obvious. If it was I wager he would have tried to safe her, since her journal implies that she was placed under some spell by Movarth the way she placed other men under her spell.