As far as I can see, the last person who would have seen Anna alive would be Howard. It's obvious because he obviously instructed Brigitte "to watch over her". Taz was in a "secondary watch" location because Brigitte noticed something wrong "first" and called her in.
We all have to question any HKS statements about when she was last seen alive especially if he ends up being the last one to claim he saw her alive. Right now it seems that Moe was the last person to see her possibly alive (someone other than HKS). Then we have Stern alone with Anna in the hotel room after Moe saw her alive last. HKS had at least 30 minutes alone with her in the hotel room. If HKS was in the room when Moe popped his head in what was HKS doing? - Was he laying on the bed with her, was he giving her meds, was he sitting on the bed, was he sitting in a chair?