While I haven't ever actually seen any videos or pics, I know enough details of my wife's PA to stitch together some nasty movies in my mind, and I'm often triggered during sex as well. "Is this how she reacted with him? Was she this excited and enthusiastic with him? More? Did she like it better with him..?"
We've always been pretty adventurous, and always have long sessions. Sometimes more than an hour. We're not really shy with each other and we're both very comfortable talking about, and asking each other questions about sex. There isn't much we haven't done. She swears it is, and always has been, better with me. I think I believe her because she's also very enthusiastic and very loud. "No. No one has ever gotten that reaction from me but you," she says, but Jesus christ. Isn't that what almost any wife or girlfriend says or does with their partner? On the plus side our sex life has improved and gone into overdrive lately, too. It's been almost every day now for over 2 months, sometimes twice a day, and it's mostly been initiated by her. The couple of times we skipped a day, she said something about it and wanted to know if something was wrong.
I'm kind of stuck on the same thing you are tho. I've brought up multiple times that it's in my mind while we're doing it. I've told her about how I keep envisioning her with him, writhing, moaning, and shouting while they did it. Him putting his hands all over her. His mouth all over her... goddamnit. It sometimes really kills the mood for me, but I manage to push through it. It just kills me thinking about it. She keeps vehemently assuring me that no one could get those very enthusiastic reactions from her but me, but again... isn't that what they all say? How do I know she isn't just saying that? I dunno. But yeah, as you can see, it bothers the hell out of me, too.
Sorry if I'm over sharing, but it's kind of my way of letting you know I think I can relate. Plus your post hits close to home and it helps getting it off my chest
As far as your thoughts on whether or not he's getting as aroused for you as he did with his AP, I can almost guarantee you he is. If he's climaxing then you're definitely getting the job done for him. Trust me, especially if your sex life has improved, you're turning him on plenty. The way you describe his actions toward you, making you feel truly loved again, putting real effort into wanting you to believe he's a safe partner again, the improved sex life... that's not easy to pull off unless it's sincere. It does sound like he's truly wanting to show you he's a safe partner again, and it seems to me he wouldn't be putting that much effort into it unless he really means it.
Even for a man, when you're in love with someone, physical characteristics aren't as important as some would make it. I think my 54 year old wife is just as sexy to me as she was when we met in our 20s, and I only have eyes for her. Don't get me wrong, she's still a very pretty girl who doesn't look her age, but she's not 27 anymore either (nor am I, lol). I'll bet you're constantly on his mind, and I'm sure you more than turn him on enough. I know what you're going through tho, and it's tough. I hesitate to say this because after a betrayal like this you should never let your guard down, but just remember that he chose you. He's with you. You're what he wants. That's what my wife keeps saying to me.
I hope the best for you guys. It sounds like you're on a pretty positive path to reconciliation. You just have some rough patches to get through still. Just like I do... sigh.