Satan in your fiction, I am sure he could. Just like Michi in my fiction could destroy existance itself and everything else in any moment.
As said about Living Tribunal:
The Living Tribunal is an omnipotent entity which presides over all realities and universes, including the main Universe, Earth X, the Ultimate Universe, and all other alternate and potential realities.
He can simply alter the reality of Kain to make him none-immortal. Just because one omnipotent being has a tough time against another does not mean they can not do anything. Real omnipotence is the ability to do anything, which from what I know is exactly what Living Tribunal can do (Save beating TOAA, which as it seems right now does not even exist- if listening to Terryc250)
You say that Marvel omnipotence is not real omnipotence. How can omnipotence be real or not? Either it is omnipotence, or it is not. Omnipotence does not change meaning, simply because an omnipotent being can not do one single thing (Like beating TOAA). Omnipotence means "All power", which in other words means that an omnipotent being holds all powers possible. LT is "all powerful" and therefore omnipotent.
TOAA might be ranked over him because the effectivity of their power exceeds his. What he does with a motion, they might do with a blink. There is more to power than just pure power.