@Ryll - that is only because Lelouch did not use the power of geass to its full extent on Orange and Viletta. If he ordered them to be his lackeys forever, it should work. We don't know the time limit of Lelouch's geass so far, maybe it does not even have one. The only time it was resisted was by Euphy, but those are exaggerated circumstances. She was also able to only resist the order to kill all Japanese for a short while, and then proceeded to massacre them despite her innocent nature. I am still unsure about her not killing Suzaku, but that was probably more for dramatic effect (to emphasize the dying character's love blah blah) than any real weakness of Lelouch's geass.
I do agree that the emperor's geass is more insidious (since it is really roundabout and deceptive), but I don't think it is more powerful in terms of achieving results in manipulating people.
Even the emperor's geass has its weakness. Even with filling the gap of Nunnaly with Rollo, Lelouch felt a certain dissatisfaction with his life, and while he did not know what it was specifically, he felt there was something off.
Also, on the last point, Lelouch can simply order someone to change permanently, and they would. He simply has not done it before. If he ordered them to think that they were chickens, they should, and if he makes the duration indefinite, it should never wear off (well, as far as we know). I mean, he even ordered someone to "forget", which is technically not something one can do voluntarily, and it succeeded.
EDIT: It has not been elaborated on yet, but I think that the emperor's geass can be used on the same person more than once. This would definitely make it very powerful, even surpassing the command geass just because it is reusable. The inconvenience of being able to use the power only once is really obvious in season 2.
EDIT2: And yeah, I started the shift in topic when I asked if the memories could be falsified, since I simply had practical usage in mind when I proposed the idea.