They really should have no problem with providing a recent valuation. You should say that you’re trying to plan out your tax liability, because depending on your jurisdiction it’s going to be taxed. From that standpoint if it’s significant and there’s no liquidity it’s actually a liability.

IMO: Unless you really, really, really want the job, it's time to walk away.

FWIW: Sometimes companies need to fill open jobs to meet artificial metrics. IE, they want to look like they're growing, so they over-hire. Or, someone gets promoted to a lofty title and now needs to hire so they have direct reports to justify the title. Use care, because if a company is filling seats with warm bodies, you will have to work with those warm bodies.

Zero. If they aren't willing to tell you what the value is, then they are hiding something, so call it worthless. Even if you disagree, you cannot get any value from them if the company is private, unless there is a liquidity event. So... Zero.