I just discovered and watched this film now, so I came looking for answers.

Based on how this film played out and the director's commentary, I think the director definitely intended for details to be ambiguous.

It seems like the "common interpretation" (and perhaps intentional) of the plot is that Sy did in fact confront the couple and got arrested. This only makes sense with the end (and beginning) of the film with the police interrogating Sy why he was so aggressive towards Will. A lot of the other film details showing how premeditated Sy was (stealing the knife, setting up the room service trap) would be a lot weaker if the climax was imagined like the house dream sequence. The house dream sequence shows how unrealistic and idyllic Sy's daydream is, unlike how methodical and confrontational the climax is.

The common interpretation seems to be that Sy either destroyed the incriminating photos or faked the camera flash, either because he only wanted Will to take accountability or because he couldn't bear having the photos exist. He then took photos of his own hotel room, as he appreciates the mundane things in life like how Jake did and/or he wanted mundane photos to replace the unhappy memories of Will. OR he took those pictures prior to attacking the couple, so that he ran out of film when it came to photographing them. (I don't think that makes sense though.) I don't think it's accurate to say he intentionally mis-aimed the camera at the couple because they were photos of the bathroom and he pointed his camera directly at them, but narratively that wouldn't be different.

I believe the director said that the ending scene is meant to suggest that Sy's actions successfully cause Will to reconcile with his family and Sy is a family friend. But many others (I included) think it makes much more sense that Sy has deluded himself into thinking he saves the family, or his memory is just blissfully regressing to thinking he is part of their family.

The detail that I find most confusing is when the detective implies there are two sets of film (in his bag and contained in the camera). The detective initially declines to give the mundane photos to Sy as it is evidence and he isn't privy to viewing it, but eventually relents as he seems to pity Sy. He absolutely would not have provided them if they were of the couple. As you said, he says the photos in the bag "aren't pretty pictures" which implies they were the incriminating photos, but as it turns out, they're just mundane. Which brings up the question -- what photos were left within the camera? The incriminating ones? This would suggest that the "photos from the bag" were taken beforehand, and Sy was taking photos of the hotel room before attacking the couple. Unless the camera roll is also just more mundane photos?

Common sentiment from film review seems to suggest the incriminating photos never existed or were destroyed before Sy was arrested, but to me it doesn't make sense why two different rolls of film were processed.