In your last photo there is a grommet a few inches above the tack. My main has a similar grommet, and it's the one that the cunningham goes through before it runs down to a fastening point on the mast. I don't think the first reef should double as your cunningham.
As for the cotter pin in the sail track.... what do you do when you douse the main? Do you leave all the slides above the pin? When the sail is up or down, the cotter pin is unnecessary. It's only while the sail's being raised or lowered that you need the pin there, assuming that all the slides that would end up above/below the pin start out above/below the pin. I would recommend that you take the pin out to reef... of course that means you can't reef from the cockpit.
The other thing that occurs to me is this: identify the slides that need to go past the pin, and tie their grommets directly to the mast (i.e. a line running around the mast, loosely enough that they can still slide freely, and tightly enough that, as the slides pass the wide part of the track, they don't pop out. You could even use some kind of rings like you see on a gaff rig. Then you can leave the pin out permanently and not worry about the slides.
1972 Catalina 27