That only added to the surveillance that was already on the Uchiha though. If the Uchiha wouldn't have already been oppressed by Tobirama don't you think the elders would have been more lenient?The coup contemplation and Uchiha Massacre occurred under Hiruzen's leadership, when the village wasn't Tobirama's responsibility anymore. And that was because the Kyuubi attack, in which Danzo, Homura and that old lady oppressed the Uchiha to a point that it was unreasonable, and the Uchiha were being discriminated against because they were widely believed to have been behind the attack. And the Kyuubi attack was under Minato's leadership, wasn't it?
They weren't sure, they were being cautious; they couldn't take any chances. How could Tobirama know that nobody else conspired with Madara?
Maybe because Madara left the village.