Jeff,
As your developing technique (i.e. wrong temperature, trying to use fixer as developer, etc.) seems to be without question, there's appears to be only four other possible options:
- Dead roll of film. Unlikely but not impossible.
- Duff camera. Unlikely for the reasons you state - and why would it suddenly spring back to life in time for the third roll?
- Duff chemicals (if a completely separate batch was mixed / used for the second film). Very unlikely if they came out of the same bottle as the first.
- Some sort of procedural issue - such as developing a roll of unexposed film. Did you process the correct roll? Do you always mark exposed rolls that are for processing or store them separate from unexposed rolls? Did you have more than three rolls of the film type you used? Could one of the ones you have left in your bag be the second, unprocessed, roll??
I had a similar thing happen to me a couple of years ago. I don't process my own so, when I got to the lab, I was greeted by a lot of apologetic comments and a completely blank, but crystal clear, roll of film that should have had some woodland / bluebell scenes on it.
All very weird and I never found a roll of undeveloped film that turned out to be the "phantom" roll.
I hope you find yours in the bottom of your bag someday soon but if not - and since you've tested your technique - all you can do is live with it and move on. If you think it might be procedural, chance your handling procedure to put exposed film in a separate pocket in your bag (if you don't already).
Also, after rewinding an exposed roll of film, never leave the film "leader" sticking out of the canister (35mm) as, if you do, you could double-expose a roll as I did once. These days I leave the film in the maker's box until I need it. All exposed film goes in a different pocket to unexposed rolls.
I'm sorry but I just can't think of anything else that might have caused the mishap.........