The speed as it is passing by the sun is the highest that it will reach in its route (the closest part of its elliptical orbit around the sun) You couldn't change that momentum to be going in the right direction (Towards mercury's gravitational influence, which is small compared to the sun) or slow it down enough to orbit mercury after it gets there without expending a crazy amount of fuel. So you could do it for a flyby, but trying to land something on the surface wouldn't work.
The other problem is since 200km/s is the fastest speed, you have to get to that point at the normal one.
Yes, the moment Earth and Mercury are the closest is when the Sun, Mercury and Earth are aligned, so going from Earth to Mercury at this moment is heading towards the Sun, but then gravity from the Sun will be too strong to be able to stop at Mercury.
The other problem is since 200km/s is the fastest speed, you have to get to that point at the normal one.