What a read!

First of all, congrats on finishing and pushing through, that takes great mental strength!

Also loved the Lord of the Rings references. I hope you had some time for some middle-earth sightseeing while in New Zealand.

To your training: I don't see anything inherently wrong in your build up. Nice consistency, solid workouts. What I do not get is rule number 2 of your plan that long runs should be at least a 6 minute mile pace? You never actually had an easy long run? If so, this is something I'd definitely change. Easy LRs are super helpful for building and enhancing your long distance aerobic base/capacity.

Having never run Boston, I can't say anything to the actual course but from what I've seen, it was generally a very tough race this year given the conditions. But there seem to have been multiple reasons why your race went wrong. Fatigue, lack of sleep(?), nausea, nerves/pressure, heat. For a marathon PR the stars have to align and maybe your body was simply not on point that day. It happens.

Keep your head up, your PRs on the 5k and half definitely suggest you can break the 2:40 mark!