Very well said. I like your ideas. It would seem that Revan was indeed someone who was willing to do anything to protect the republic. Yet what interests me is his combat effectiveness. I mean for one thing during the Foundry battle he uses a known Jedi Knight ability known as master strike yet he also uses lightening. As such would this not make Revan a true "Grey jedi" yes he seeks the destruction of the empire but at the same time he seeks to shield the republic and defend innocents.
Now perhaps it is true that the philosophies are too entrenched in their beliefs to be used together. But you did state that a "serene" Sith is still very highly plausible because there are examples within the lore of Lords who have achieved this serenity. If this is the case then as you say the Jedi could use the example of Windu which is to teach a very advanced form of combat which is aims to push Jedi to their emotional limits but at the same time teach them the strength to refrain from giving into the urge for battle.
Also another example of how this could potentially work in the battle with Sidious Yoda was able to push back and counter the lightening that was being used against him. That to me indicates that Yoda must have done some learning into the abilities of the Sith to be able to combat them. So if Yoda was able to still maintain control yet have the ability to use Sith powers then i still believe that with enough training a Jedi can safely learn certain Sith arts. If anything it would be strictly controlled and it would only be for martial training.
The only problem with this is that I believe only a certain skilled few could manage to accomplish such a balancing act. Mace Windu did create a way to channel his dark urges, but almost every other Jedi who tried to master his style became corrupted by the dark-side and had to be put down, and Yoda is one of the most powerful Jedi in all history. A small-time Jedi wouldn't be able to handle the constant influences of power and stay vigilant.
Kyle Katarn was a Jedi who tried to use both sides of the force, at first believing that it was only how you used it that made a power good or evil. He later gave that idea up because he found that the more you used a power that relied on a dark emotion the greater that emotion became over time. If you're constantly using your anger to unleash some power then anger becomes something you can't bear to get rid of. You begin to believe that you're only strong when you use those dark emotions, and from there it's a slippery slope to the dark-side.
A Sith could gain some advantages over others by embracing some light-side techniques, but once again it would begin to influence how you thought. Eventually they would lose that drive to gain power and become an easy target for other more ambitious Sith. Malgus knew that no matter how much he loved his wife she would always be a weakness to him, so in order to gain power and lose that weakness he killed her even though it broke his heart to do so.
Once again I believe that a select few individuals could possibly create a code utilizing both orders, but they would have to create their own organization and recruit like-minded people instead of doing it from within the Jedi or Sith.