He definitely didn't find Obito by chance, a simple look at the maps explains that. The distance between the land of Grass, where obito got hit by the boulders, and the Mountains Graveyard, where Madara was, is almost as much as the length of the land of fire. I certainly don't think his lair is that big.
Of course, his "Brilliant plan" Colapsed after about a second. he didn't have a singly (Effective) backup plan incase Obito betrayed him. Sure, black zetsu. Obito beat him, and the ten tails, with a hole in is chest.
And of course, his plan couldn't work if it had gone perfectly. Blind Old Man vs Ten Tails.
Of course, his "Brilliant plan" Colapsed after about a second. he didn't have a singly (Effective) backup plan incase Obito betrayed him. Sure, black zetsu. Obito beat him, and the ten tails, with a hole in is chest.
And of course, his plan couldn't work if it had gone perfectly. Blind Old Man vs Ten Tails.