Interesting and potentially vital update; while messing about with tailcap i first went with the same grease that HDS sells with their replacement o-rings since i wanted it to be slightly firmer, in the pics above i instead decided on trying Nitecore's lithium grease on the o-rings just to see what'd happen. 20 minutes after putting it together it had gotten noticably stiffer, an hour after it was hard to twist the tailcap overall. Fearing contaminants in the grease i disassembled the tailcap again, taking it apart i noticed the o-rings being slightly more snug in their slots this time around, having that in mind i wiped everything clean and added the first grease again. At first, no issues, then after a few minutes it had started seizing too. A few more attempts with this, also trying Haynes lubri-film, the o-rings had gotten larger time by time. This morning after sitting a few hours the tailcap required considerably more effort at first, even if it gets slightly better with repeated twisting.
My conclusions based on this and when the issue started appearing is that the material in the o-rings started expanding after coming into contact with the Nitecore grease, leading to them swelling up in place and acting more like a brake instead of a glide surface. I'm getting some new o-rings and will try more grease on them to see what happens, if it's not just this material being iffy. I can't recommend using the Nitecore grease for this application as it is.