Can I safely install a battery kill switch on my Nissan Pathfinder

That's just not practical , it could be the ignition switch it has to be diagnosed , You were on the right track with the milli volt draw you just have to do it correctly ,

Doing this will eventually cause computer troubles, everytime you disconnect main power your forcing the computer back to factory settings, it may one stop doing this and then you at the dealers mercy to bring everything back and that could mean a new computer. You need to find the draw, check the rear wiper, I have seen them have a draw cause they don't park properly.