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I'm sure you'll get some additional input on this; I'm guessing that the engine was installed "all the way down" on the transom base on your description of yawing / torque roll. Having too much of the lower unit in the water at speed will increase drag and can cause this effect. The diameter and pitch should be stamped on the propeller hub, let us know what information is on there. Your RPM readings are WAY below what Evinrude recommends for the engine, the propeller on the boat may have too much pitch and will cause damage to the engine if you run it under these conditions over the long term. I'm guessing by the numbers that you have at least a 19" pitch, maybe even a 21".

I have the 2012 version of the 90 HP E Tec on my 1989 Montauk 17 with a 14.25" x 15" pitch Stiletto Advantage stainless propeller, engine is mounted "two holes up" on the transom and will turn 6,200 RPM / 43 mph on GPS with just myself on board. The Stiletto propellers have been discontinued although they sometimes are available used. If I remember correctly, another choice that was recommended when I was repowering was the Evinrude "Viper" stainless propeller, 17 inch pitch. (The Stiletto propeller I have acts like a 17" pitch even though they called it a 15") There's a ton of information on here about propellers, transom mounting height, etc., do a search and you should find the answers you need. Good luck and keep us posted.