Greg, I’ve done that research ..exhaustively. Your list is good. I use Willis because 1) a bit cheaper 2) they endorse all major dive cert agencies (I am PADI, NAUI, HSA) 3) service has been EXCELLENT, very personal.
They did NOT pay me to give them a plug (lol)
And Greg you’re right, endorsed=commission
Even if teaching only friends & relatives or occasionally your exposure/risk is the same. They will not limit the number of people you teach in order to create a discount. There is a way to get a better rate if you are attached to a dive shop with a group policy. That confines you to teaching only for that shop (they make the profit). Last I checked group premium/person is around $300/yr. The other way is to teach only non-Americans in a country that does not require insurance. Then PADI will let you sign a dozen more waivers to be acknowledged as exempt.
My Opinion; teaching friends/relatives is the HIGHEST risk. They often make the worst students & if anything ever happened they would want you to be totally responsible for everything. Just reality talking here. Over the years I have gotten girlfriends & eventually my wife (now ex) certified. I paid for another instructor to teach them. I’m sure it can’t be all just me! lol!!
Good luck guys.