I'm calling it now: he's a widower who started watching miyama's porn because she resembles his late wife, and it became a way for him to cope with the loss.
bonus theory: his wife and tamagawa are related somehow, and that's why they look similar.
I guess I'm the only one who think Manager like Miyama because she reminds him of his former wife who (I assume) died and Miyama is cope from losing wife. I thinks more tragic than broken marriage due to porn addiction.
If the FMC were Miyama Yuri, a former actress who was so popular in her time, more than one person would have recognized her in the store or anywhere else. I don't know... something tells me there's something fishy going on. Miyama Yuri's friend said at the bar that she couldn't confuse Miyama with anyone else since she was her only friend in the industry, and yet she says she's not sure it's her. A simple change of hair color shouldn't make you doubt whether she's your friend or not.