AMT ISO deferrals for nonimmigrant. Sold shares after leaving US

• I lived in the US from 2013 and 2016 on an E-3 nonimmigrant visa.

• As part of my full-time employment contract, I was granted ISO shares in a pre-IPO company.

• I exercised most of the vested shares in 2015 and 2016. For these
  years, I paid AMT due to the exercise events.

• In 2016, I filed my last US tax return and left the US permanently to return to Australia.

• The company went public early in 2019

• In late 2019, I began selling some of my shares. I understand this is a "qualified" ISO sale, and long-term capital gains tax rules apply.

I’m now attempting to file my US tax for 2019 and have tried to work with multiple tax agents. Each have tried to do my tax return but all have provided wildly varying figures for how much I owe the IRS. One of them even did a $0 return… which didn’t add up. I suspect the challenge is due to the added complexity at the cross-section of ISO deferral items (AMT) with the change-of-residency issue.

Can anyone help me understand how to calculate my 2019 tax return, or provide advice for how I can find an accountant who can give me confidence that they know what they’re doing? How do I ensure my AMT credits are properly used and carried forward? I just want to pay the right amount to the IRS – no more, no less. The only idea I have is to get the tax agents to talk to each other and peer-review each others returns… but I feel like this is an unnecessarily expensive exercise.

Bonus question – in the future if I make capital gains over the threshold, will I be subject to the Net Investment Income Tax (NIIT)?

Thanks!