...and then explain exactly why it is that an AI cannot have a fixed goal architecture.

He answered that in the interview.

He answered, and asserted it but didn't explain it.

Wang needs to show that AI's with unpredictable goals are somehow safe...

He answered that in the interview.

He answered, but didn't show that. (This does not represent an assertion that he couldn't have or that in the circumstances that he should necessarily have tried.) (The previous disclaimer doesn't represent an assertion that I wouldn't claim that he'd have no hope of showing that credibly, just that I wasn't right now making such a criticism.) (The second disclaimer was a tangent too far.)

He wrote that AI's with fixed goal architectures can't be general intelligent and that AI's with unpredictable goals can't be guaranteed to be safe but that we have to do our best to educate them and restrict their experiences.

The latter claim is the one that seems the most bizarre to me. He seems to not just assume that the AIs that humans create will have programming to respond to 'education' regarding their own motivations desirably but that all AIs must necessarily do so. And then there is the idea that you can prevent a superintelligence from rebelling against you by keeping it sheltered. That doesn't even work on mere humans!

...and then explain exactly why it is that an AI cannot have a fixed goal architecture.

He answered that in the interview.

Yes, but the answer was:

If intelligence turns out to be adaptive (as believed by me and many others), then a “friendly AI” will be mainly the result of proper education, not proper design. There will be no way to design a “safe AI”, just like there is no way to require parents to only give birth to “safe baby” who will never become a criminal.

...which is pretty incoherent. His reference for this appears to be himself here and here. This material is also not very convincing. No doubt critics will find the section on "AI Ethics" in the second link revealing.

What makes you believe that his expected utility calculation of reading Bostroms paper suggests that it is worth reading it?

Nothing. That's what he should do, not what he knows he should do.