But that only works if Congress has an incentive to vote to impeach. If we're in a hypothetical situation where we know the president to be a foreign agent, ass

But that only works if Congress has an incentive to vote to impeach. If we're in a hypothetical situation where we know the president to be a foreign agent, assuming that people in Congress are not similarly compromised, at the very least they are members of the same political party as the president. Impeaching one of their own would have political consequences, and Trump's approval rating still shows a pretty decent base who like him. It would be basically political suicide for any Republican to vote to impeach because many Americans don't believe Trump is an agent, don't believe he did anything wrong, and still agree with his policies and agenda. Congress has the ability to impeach, but not the will.