Should (or even Can) I use epoxy to repair a previously PVA-glued load-bearing wood joint?
I think you are over thinking this. 

I made this exact same repair a month ago and I used wood glue, you could use construction adhesive. Sand both surfaces, and glue and clamp. 

Add extra screws for strength, one every 8 inches ? . 

No need to fill in counter sunk screw holes, with box spring/mattress in place that part of the frame in not seen.