There are a number of Yams designed to run on 100/1 premix and have no less of a life than those designed to run on 50/1, or on auto-lube which in some cases has a ratio of 200/1.
Where in the world are you? I have no idea what RNLI means.
When you say "I had the neutral switch bypassed on the Evinrude" I am presuming that you mean the control box neutral safety switch was bypassed. Not a particularly smart idea but so be it. To each his own.
There is no wire to bypass in a Yamaha motor. If battery power is provided to the motor...
I remember the days of 16/1. Folks were scared to go to 25/1 and added extra oil.
Then they went from 25/1 to 50/1. Folks were scared to go to 50/1 and added extra oil.
Now they go to 100/1 and folks are scared to go to 100/1. Adding extra oil.
It is not abnormal for there to be a period of time before water starts coming out of the tell tale. A minute's time would not concern me.
Having said that, when was the last time the water pump was replaced?
Is the rectifier installed on the motor in addition to this mysterious part?
My guess is that it might be an upgraded rectifier or perhaps a rectifier/regulator. But...
Well as you see from a comment above, some don't feel the need for a temperature gauge. It would be a waste of time and money for Grady to spend time and money installing equipment that some don't want. That is why it was was offered as an option.