I think Columbo has a decent circumstantial case against Grace, but I agree there is possible room for reasonable doubt.

I don't remember Raymond making any reel changes. I remember him occasionally stopping in to see if Grace needed anything, but I thought the idea was that the whole movie was on that one reel. I don't know if that is true to real life, or should I say "reel" life. Maybe prints of movies available for home viewing were done more cheaply and everything fit on one reel?

He makes a reel change after Grace commits the murder. Movies back then always consisted of multiple reels and you couldn't have it all on one reel unless you had a giant sized projector.

Even apart from the reel changes, broken film etc, I've never understood how watching the movie on her own would have given Grace an alibi anyway.

She technically didn't think she'd need an alibi since the idea was to make it look like a suicide based on the fact that the door was locked from the inside.

I just watched this episode again last night (it has suddenly appeared on Netflix, along with all the other random episodes that they were holding back on). This plot hole occurred to me, too, and I was specifically watching for it. Immediately after Grace splices the broken film, Raymond does appear, presumably to change the reel. But we don't actually see him change the reel. He pops into the projection booth and then reemerges. In fact, he couldn't have--the reel would not have been at the end, because of the lost 15 minutes. So he must have noticed that there was a lot of film left on that reel still, and left and came back again a little later to change it. From that point forward, the reel changes would come at regular intervals again.

Why would he not think anything odd? Perhaps it was not unusual for Grace to stop the projector herself for a lengthy bathroom break every now and then.

Yeah, it's an entertaining episode, and plot hole-forgiveness not withstanding, the stretch for me was in for as many times as he ran the film for Grace, that Raymond didn't notice that "Walking My Baby" was still running when Johnny Carson had already ended.