Yes, I know that Lelouch can only use his Geass once per person, but can he control everything? I mean, if he says "I command you to do whatever I say", will it work?
Yes it would, and he have done so on multiple occasions.
(For instance, on Schneizel, "Obey zero")
Lelouch has done so several times. One of it is like Madara said. Here's another examples:
- To Kururugi Suzaku, "Live!"
- To Gilford, "Whenever you see Zero putting his hand on his collar, you will see him as Princess Cornelia."
The limitations is of course the physical limitations of the subject of the Geass power. For example if he ordered someone to go to the Moon on foot, then it will not happen.