Nice job spending half your post on an irrelevant misdirection and proving you don't understand the definition of "potential" either, Nai.
Point: Leia, Luke, and unborn Anakin had to use their full potential to defeat Sidious. Understand what that means? They used what power they could have realized to its fullest. In other words, they could never exert more power.
Given that, no matter how much training and knowledge Luke has achieved by DN, it doesn't matter - his full potential alone is not enough to defeat Sidious.
There's also the unfortunate (for you) fact, that he had access to a complete Jedi training facility (the Chu'unthor), several Jedi holocrons, and parts of the library on Ossus. And yes - singlehandly destroying an entire army of beings that have lightsaber resistant armour and weapons, destroying their elite warriors and their toughest warrior - when all of those beings are completely immune to force attacks (so much that the force lightning arched away from them) is pretty impressive.
Chu'unthor - Any living, breathing instructors of lightsaber combat aboard? I thought not. I can learn to fight with a sword from a book, too. Doesn't make me any good. I can learn to fight with a bunch of other neophytes too. Say I kick the shit out of them - does that make me a god of combat? Thought not.
Jedi holocrons - Unless they projected living instructors of the weapon (oh, and you'd have to prove what the holocrons contained, by the way. Saying "zOMG HE HAD HOLOCRONS" is not an argument) your point is rather moot.
Ossus - Yeah. "Parts of the library" that got fried by a supernova. "Parts of the library" that was, in its prime, so enormous that the frenzied efforts of hundreds of Jedi to rescue the precious knowledge therein "wasn't enough." Oh, you also run into the problem of proving what he learned from it, again.
Killing the Vong - Again, you only mark this as impressive because other so-called "Jedi" got their shit ruined by them. You've still failed to establish that real (i.e. PT and earlier) Jedi would have similar results.
And I love how you ignore that some of Luke's students and Luke himself did train under and defeated actual "living PT Jedi".
Okay, let's go down the list then!
Kyle Katarn tooled people that were trained in PT times (Jerec)
Tooled? He tooled Jerec? Yeah. That isn't biased language.
Yeah. That somehow means that Kyle "I found a lightsaber in a garage and that makes me a Jedi!" Katarn is great, rather than that Jerec sucked ass.
Mara Jade
Oh, right, the same Mara Jade that lost her already-weak Force senses after the Emperor died? The same Mara Jade that was trained as an assassin who rather than a full-fledged lightsaber duelist (indeed, often using other weapons than a lightsaber)? Yeah. She's a shining star of Old Jedi Order combat skills.
Kam Solusar
Oh, you mean the Kam Solusar who "lacked formal training" under the Old Jedi Order, ran away for thirty years, then came back and was captured and converted by Sedriss? Yeah. Because he had a lot of formal training under Palpatine, I'll tell you that right now! You're not just hocking some unestablished bullshit!
Oh, didn't you say Kam was trained under Sidious and Vader? How, may I ask, was Kam trained if Vader was dead by the time he returned to the galaxy proper?
Vima-Da-Boda
Again, what lightsaber combat was she teaching, hm? I seem to recall her giving her own weapon away. Looks like you're still not establishing a damned thing.
And Luke's saber skill is above them all so much that he thinks (DN) that he can fight as good as all other Jedi using his offhand
Not surprising inasmuch as he trained them all (minus a handful who have about as much or less experience than he does).
and he killed Lomi Plo, who nearly killed Mara and Jacen both at once, with "a flick of his wrist" in a lightsaber duel.
Hm. Insofar as I can see, neither of them are PT Jedi. Still don't see you establishing dick about Skywalker's lightsaber abilities.