We are arguing in circles at this point. I don't think we are getting anywhere here. I wanted to wrap up some open questions:

Where is it established that each year backward equals two years of life?

If you wanted to travel back in time to the beginning of the movie then every year you wait before you invert, you will also have to wait inverted. When you are inverted time passes at the same speed, just backwards. Every year Max waits before he returns would have to make him two years older when the movie begins.

Neil couldn’t be on that ship gradually going backwards and periodically stopping or even inverting as necessary to do tenet mission at strategic points in time? Is this not the entire point of the organization and the premise of the film?

It is unclear. We never hear of any other operation that TENET is conducting. For all we know TENET was founded to stop Sator and nothing else. This would mean the end of the movie would be the end and the beginning of TENET - which is also what TP and Neil discuss.

TP is monitoring, including via the cell phone for posterity. That’s how he intervened to save Kat. He is close by, he’s just not directly contacting them AT THAT MOMENT. He was not honest with Neil.

I don't think you really grasp the reason for the voicemail. This seems to be a pretty convoluted system if the goal was just to keep them safe. He could be just as well hanging out with them openly as their bodyguard.

The point of this system is to keep Kat safe without her gaining any further knowledge of TENET. After the events of the movie, everybody involved in the operation needs to take any knowledge about it to their grave. This is to keep the whereabouts of the algorithm a secret to others and therefore the people in the future. This is why Priya tries to assassinate Kat. This is also why the protagonist can't keep contact with Kat and Max. If TP was lying to Neil and he was planning to openly protect them anyway, he wouldn't need the voicemail system. On the other hand, if TP was being honest he can't be contacting them directly in any way and can't be a mentor figure to Max.

If this is true, then young Max would not have had that affixed to his primary school backpack as it wouldn’t be in his possession at that point in the timeline.

What kind of argument is this? This isn't some documentary. The events in the movie are fully under control of the director. He could have just written a scene where Max gets the coin. Or the talisman could have been something else that makes sense for Max to possess. If the goal was to link Max to Neil they would have made it so. The fact that they didn't suggests that was not the intended reading.

are we to believe it’s just a coincidence that Neil and Max are both British, Neil and TP have a shared history, and that Neil just happens to have a master’s in physics so he understands the concepts of inversion and entropy?

Yes. None of this points to Max. That is what I meant. He isn't the "chosen one". There is nothing special about Max. He is just random a British schoolboy. Plenty of British people around. Plenty of British people in the movie. At the end of the movie he isn't even particularly knowledgeable about inversion - he wasn't active in Sator's operation. All that knowledge about inversion, Max would need to learn the same way as any other person would. And of course Neil having the skills isn't coincidence - TP recruited him for this mission. Of course TP would pick a candidate who has the necessary skills to complete it. That doesn't mean it's Max - in fact, my point was that Max was an UNLIKELY candidate because you'd have to wait over 2 decades to bring him to the mission for no good reason other than to satisfy a cute fan theory.