How could it be bud rot when it showed face over a month before flowering even started? Bud rot is mushy and has spores, this doesnt.
How big are mites? I honestly didnt look super close for tiny incects. Its feet away from a 6ft tall swiss cheese that is like a month in to flowering flowering and it is georgous. That one is almost impeccable other than a few very small dying brown spots on the bottom like that shit plant. I feel like its something in the soil but im not a expert.
What would you like to hear?
It's bud rot (or at least it appears to be). Your other plant will probably get it too (probably already has it, just isn't consumed by it yet). Maybe the area you planted in has a high RH...maybe air flow in that area isn't great. Maybe your soil stays too wet. Maybe it rains too much. Maybe that plant was weak and took the brunt, saving the bigger healthier plant. Maybe that's wishful thinking.
As far as I'm aware, there's nothing you can do once it's this bad. Next time make sure you have plenty of sunlight, good airflow, and quality soil. Trim your plants up away from the ground and consider a cover if you get heavy or sustained rains.
But for now, I think you're fighting a losing battle.
For sure prune off any brown crap from the plant you still have hope for. Hopefully you get a harvest of some kind still.
Good luck