I pretty closely skimmed your post (gtg for now!). I’ll come back and read it at a more leisurely pace soon.
In the meantime I wanted to say this: why are you looking at an external blower? It seems to me that integrating a blower inside the unused space of the battery tube makes the most sense. The concept of being able to convert back to a fully-handheld/no-external-batteries light seems like nonsense to me. This a seriously powerful build and will require a remote battery pack to be useful/interesting. You may as well treat it that way and install a blower internally.
There’s plenty of space in there. Here’s how I think it could go down (but I don’t own a BTU Shocker): If you modify how power goes through the head and into the battery tube & minimize driver size you’ll have enough room to drill/cut vents at the bottom of the fins. After that you route the power wires as far off to one side as possible (top side, towards the handle). Use a small blower with the intake facing the front of the light, then seal the tube around/behind it. Vent radially around the blower. You’ll only be able to fit maybe a 30-35mm blower in there, but I suspect that would make a big difference. You could also look at using an axial fan with a high static pressure in the same configuration.
That’s all my two cents and it’s not my build of course… it’s your show! I just noticed that you hadn’t mentioned considering this option and kind of wondered why.