What color is the Instant Mytol concentrate? In my experience, such glycol formulas starting turning yellow after 6 months, and then become orange, and then reddish, all indicating oxidized developer that is becoming weak. Glycol developers last longer when stored refrigerated or frozen. How did you store yours?
High speed film tends to fog over a period of years, and I see much fog in your filmstrip. That tells me the Instant Mytol was performing tolerably well.
Also, film experiences latent image decay -- the slow loss of the image.
All told, I think your development went surprisingly well given the circumstances, and I think you'll be able to get some presentable prints from that film.
Mark