I don't think color has any impact on Dark/Light side standing. It's just when you first pick your saber from the council, it determines how you gain your force powers. My character has been using a Red blade for a while, and there has been no dark side points given to me. Could be wrong, if you're already a dark sider, it might boost, but I doubt it.
Secondly, what do you mean by "unlimited"? I got all the crystals in there, and after that it wouldn't let me get any others in the cave.
Thirdly, Duke, I'm afraid I don't have any set path to go to get to the cave, i was just wandering and eventually found it. It's in the back part of one "area." I THINK it's an area next to the Dark Jedi there, but I don't know. Look EVERYWHERE, Go to the border on every area. It'll take a while, yes, but right now it's the only suggestion I can give.
As for a double blade, yoink Bastila's. And if you hate yellow, once you find the cave, you can put green crystals in it. There will be double blades available later, but none on Dantooine as I recall. Again, could be mistaken, wouldn't be the first time I missed an item like that.