> Has anyone else experienced a similar situation? Am I in a no-win scenario? Can or should I stop the portions of the job that go above and beyond? How can I bring this to a peaceful resolution without delivering an ultimatum?
I saw family businesses of friends lose people who've worked with them for years without them telling them anything, maybe because those people experienced what you're experiencing. They would always tell me this bit: "we wish they approached us and talked, or at least some heads up that they got a new job".
You've hit a wall in your career, you're relying on spoken word from men instead of signed contracts - which is quite dangerous for your career path. Unless you're in it to actually develop and grow a long and deep relationship with the people, not the company, talk with them.
They've talked you into this position and you didn't sign anything, I think it's fitting to also talk to them as well, just tread tactfully. 10 years and that new "role" is enough to assume that they've come to embrace you as a key person in their father/son company and they would, hopefully, professionally hear you out.
You'll need to include them into your thought process. If you were to stop portions of the job on your own, they'll be quite surprised and it'll put you in really bad light. I mean, you've been working cleanly for 10 years and you suddenly changed after those months into the "new role", then they'll know something is amiss.
Also note that if you look for another job without telling them anything, it also puts you in a bad light. If they've slowly come to embrace you as family during those 10 years, they'll want to know why you're leaving.