It is said that Shodaime defeated Madara in a fight at VOTE, but didn't necessarily kill him. Shodaime's cause of death is still unknown to us if my memory's serving me right.
Maybe madara killed the first later and no one knew it was Madara who killed him. This may be how he gained MS, however I don't believe the whole kill your friend thing.
him stealing them is very likely, but it would have been stated earlier that all of their eyes were removed or their bodies were missing after the massacre.......but its likely he stole some from shisui which itachi supposedly killed
Doubt it. I think they would have noticed that when they found him, they are likely his eyes.
it's probably something you can do after you go blind in one eye.
I believe the whole MS eyes deal is meeting the condition of doing anything for power, meaning it is getting to a certain state of mind-insanity