Pretty much all of the Nitecore PD's need a good cleaning and relubing. The flickering is from some kind of electronics bug and luck of the draw. They issued a newer version which has 4-5 big black IC chips visible on the back, vs 1-2 for the older version. That supposedly fixes the flickering problem, but I only have the older style with no flickering so I cannot confirm.
Mine appears to have 1 IC chip visible.
D10's are not well regulated at high levels with alkalines. Most people are using NiMH or Li-Ion.
Yeah, in the works but get alkalines free from work. Yes we do recycle them though.
The light turning on by itself is most likely due to a sticky piston ring. When you twisted it off, the battery tube broke contact making the light turn off. However, the piston ring was still stuck in place and later on, it decided to unstick and pop back. If the battery tube was not unscrewed far enough, the piston ring will pop back far enough to re-establish contact with the battery tube and turn the light back on. I had this happen in my backpack once and it was VERY hot when I found it. Opening up the piston ring with a small screwdriver will help. I don't think the spring assembly is accessible on the D10, but on the EX10, you can clean and relube it. Aside from these modifications, I always give my units a good slap after turning them off to make sure the piston ring has fully popped back. Good luck!