Let's go over things point by point.
1) Range damage, I already stated in another report that Final Strike ignored racial/size reductions but was affected by Long range reductions.
2) No idea on Nova, but on kRO Final Strike always deal a minimum amount of damage, even if your HP is 1.
3) If you're Fujisawa on Discord, I already told you the Mirror Image behavior is the same on kRO.
We might need to look at damage numbers and how bonuses/reductions are calculated @Nova