This an amazing thread but I stopped reading around page 23 so if I'm repeating, sorry but even I can only take so much Lore debate.
I'd like to suggest a simple definition of sorts to Jedi and Sith alike. Each follows their respective code and draws on their respective side of the force.
It's broad, but I think it covers all the various sects on either side as the respective orders change while still being particular enough to clearly denote where any given individual would fall.
This allows for the LS Sith and DS Jedi which people play as while maintaining their allegiances to respective factions.
For instance a LS Sith could use the DS of the force for (debatably) positive results a la Utilitarianism. They use the power they gain through the DS and fuel their force powers with emotions forbidden to Jedi in order to produce net gains.
Since so many people are clamoring for 'canonical' proof a LS Sith that does not become a Jedi I'll through out a possible contender.
Darth Cadeus. After killing Mara Jade (150 DS points), we can safely say that he was a Sith. He spent years of studying all force sects (read: acquiring power) and began to use his emotions to fuel his power. He did everything to try to protect Allana. If you're following, I've just shown he's your average Sith and very Darth Vader-esque.
Now the twist: He did it all for the greater good! That's right, he killed his own aunt, took the mantle of a Sith Lord, and plunged the entire New Republic into chaos trying to make the galaxy a safer place.
I won't debate whether or not he succeeded (not sure this can be debated given continued plot developments), but I think the intent was clearly there as iterated when Luke and Ben later discover while they search for Abeloth.
That's proof that a Sith can do good while maintaining the Sith code.
In Revan (novel), LS Revan post awakening uses force lightning to kill a Sith Lord (name escapes me). Proof LS uses can use DS powers.
Combine these two conclusions and what do you get....
SHADES OF GREY!
Sith are not more inherently evil than Jedi are inherently good. There are established doctrines which clearly make it easier for a Jedi to be "good" than Sith, but its not an impossibility to see a moral Sith. Would it be hard to resist temptation while using the DS for moral ends? Absolutely, but not impossible.
I'd like to point out that I decided to make an ends justify the means kind of Jedi that is technically DS. I save innocents all the time, but occasionally killing someone to save a whole lot of people is DS for BW...
Food for thought