I know this is a semi-old post already, and my reply to it totally off-topic, but I wanted to say that I really like your post. I always find your contributions valuable, well thought-out and insightful.
Anyway, I wanted to chime in that, as you already know, LSD is supposed to interact in a very specific way (compared to other psychedelics) with a specific loop in the 5Ht2A receptor, which augments it's "occupancy time" on the ligand binding site, which is thought to explain why it's so uniquely potent and long-lasting. So yeah, binding affinity may be lower than compared to NBOMes, but it activates the receptor for longer and more persistently, which might mean that in practice it is still more potent.
I like your hypothesis, though, that NBOMes were actually being taken in huge overdoses. I mean, specially for a drug that is so selective for a specific subtype of serotonin receptor, it makes sense for me to think that you could push the dose higher and higher, and some effects will be getting stronger without having a huge amount of general "psychedelia", like the sum overall effects can still manageable while only some of the effects are getting stronger. Makes sense then that people would be pushing the dose to absurd levels.
I bought a sizeable amount of 25i just a few weeks before coming across the first report of an NBOMe death I encountered. I was scared to take it for years, but I still kept the solution I had prepared stored away. Recently I diluted it 100x and then flushed it, it was the first time ever I did something like that. But I still stashed away a couple of doses for my personal drug museum lol.