When sizing extension wires remember your calculations are for round trip, not just one direction....most trolling motor calculations are for a max % 5 voltage drops at max amp draw.
According to Minn Kota when adding extension its a one way trip. I am thinking they are the experts and when their chart says when adding a extention up to 15ft use 8 gauge wire, that is what a person is supposed to use. Extending the distance from TM to battery 15 ft according to Minn Kota you do not figure 2x15 for 30 ft. From what you say if you add a 15ft extension you figure 2x15 for 30ft. That is what you are saying isn't it?
Dielectric grease is a surface protector and not electrically conductive. It's generally silicone based, clear and less messy than No-ox. No-ox is electrically conductive and can be used within connections. It never really dries and looks similar to anti seize products. But, whatever works for you.Rg
TOTAL CONDUCTER LENGTH... that's the way it's measured and the way it has always been measured, re read your installation instructions...you also need over current protection in the form of circuit breaker of fuse , but if you think about it in a logical manner it will make sense, the most common electrical problems in a boat all begin with undersized conductors and improper termination.. ...as well as a total misunderstanding of rules and principles that apply.. do it your way... good luck.