They do, but you sell forward contracts instead. This is perfectly legal, and the approach I've seen. There are a few companies, and even funds that will engage in this, in an effort to attain future upside.

Its pretty common, Ive personally done this.

Typically. I’d be shocked if OpenAI let employees sell their options like this without requiring approval

They don't but you effectively do it under the table

Do they sell at full value?

Whatever they sell them for is the value.

How do you define value without an IPO?