This is vague recollection, since you don't seem to be making any usable progress.
Biff made a video for the first i3wm microwatt. Updated it, I think, for R10. I'm pretty sure that he covers - later stages, perhaps - the fact that config is all in an editable text file. I do realise that you want to allocate the MOD key itself given that none of the existing key functions that way, as opposed to binding mod+key combinations. I say start with the video.
Depending on your typing habits, consider a right-ctrl key as mod .
Good luck - after this amount of effort you deserve it.