If you exercise shares in a dark market why don't you just mark them at zero? If they aren't trading you can just make up the price and that goes for any good not just shares of companies.
The 409A valuations are real and there are rules surrounding how exercise happens. You can't just sell them outside of that and not pay taxes. You could theoretically sell the shares once exercised to some other private investor if you can find one, but you'd still have to follow the same exercise rules.
That's why shares that trade openly are called 'public market' shares.
Reading this thread is fun, it's like trying to see Silicon Valley try to reverse engineer something hiding in plain site.
95% of these problems would go away if these companies were forced (by investors or employees) to go public.