I can get down to just under 2MPH with my 115 HP Mariner at 600 to 700 RPM, same thing with a fishing buddies 90 Mariner. There should be no problem whatsoever getting down to 2 MPH on any make 50HP both 2 and 4 stroke. The key is how well idle speed is set up and how well the outboard is tuned. I have set the idle while under load to get the minimum trolling speed. It is hit and miss doing it on the muffs. Mercury should be no different than any other makes.
I just noticed this is your first post, welcome to iboats Hodor, the best boating forum bar none. Hang around, you will thank yourself. I will hold the door for you.