You can do Sekka -> Evisceration -> Exenterator -> RF -> Evisceration for Distortion -> Darkness
We can't really 2-step Fusion or Gravitation, which limits our options pretty badly. If Exenterator had been Fusion and Dancing Edge / Shark Bite's skillchain properties were reversed, we would be in business.
I think you mean Sekka -> Exenterator -> Pyrrhic Kleos (Distortion) -> Reverse Flourish -> Evisceration (Darkness)
Evis -> Exent is Fragmentation, and like you said we can't do fusion so unfortunately that kind of dies there if you want level 3.
If you have another person with a frag ws, or same setup with a TP wing... you can do:
Sekka -> Evis -> Exent (Fragmentation) -> Tp wing -> Pyrric Kleos (Distortion) -> Evisceration (Darkness)
The fragmentation has 93% skillchain damage with skillchain bonus and Charis Bangles +2
The distortion has 108% damage with skillchain bonus and Charis Bangles +2
The darkness has 208% skillchain damage with 4 skills, skillchain bonus, and Charis Bangles +2
So ultimately if none are resisted the skillchains add an additional 409% Weaponskill damage.
With someone with a fusion weaponskill, can do Pyrric -> Their Weaponskill (Fusion) -> Exenterator (Light) as an additional option rather than just Their Weaponskill -> Exenterator (Light)