Alright, I'm still fighting this, and I'm needing to come back for ideas. I put all new seals across the back of the camera last night. I used this as guidance (
USCamera Light Seal Placement Guide | Ricoh XR7USCamera ) and cut and placed a seal just like what they're describing. The only place I deviated is that I used 3mm thick foam at the seal on the right hand side next to the take up spool where the door hinge is. This is what I used in that same spot before and didn't have problems so I figured I would do it again there.
I loaded a brand new roll of Arista Edu 400 (it expires years from now, I got 10 rolls last week and they went straight into the freezer when they got to my house) into the camera last night. When loading I put the film leader into the take up spool, shot one frame, advanced to a second frame and closed the door. The door feels good on the open & close, I don't think there's any hangups on the seals, and I don't see any of the shiny backing showing (so the seals aren't twisted or anything like that, that I can tell anyway). I waited until today after work so I could take the camera outside and shoot in sunlight. I developed the roll this evening and hung it up for a bit before taking the following pictures. This is not what I was hoping to see.
Sorry for the reflection but it's not really hiding anything. This is the start of the roll. There's a jet black section at the start of the roll that seems like it would be a few frame's worth of film. Then, looking at the film with a bright light behind it, there's a very dark grey section that runs for about 5 and a half frames. This has been extremely consistent across a number of rolls that I have developed lately which were shot over the last year or so. I'd call this 8 or 9 frames taken out by whatever is going on here.
This is the next section of frames which have discernible exposures. I see fogging along one edge through most of these frames.
This is the end of the roll. Things seem fine here all the way around these frames.
What's my next step? I have a couple other 35mm cameras I could run a roll through if there's a question about my technique in handling and developing film. The camera I've been using seems like it's shifting shutter speeds properly though I don't have a great way to really check it myself. Batteries are not very old in it (electronic shutter in this camera). The extreme repeatability of these issues across rolls makes me think this isn't a light seal issue (besides perhaps the fogging along the one set of sprocket perforations). The things shown in that first image look exactly like the previous rolls I've developed.
Thanks again.