What would happen if you decide to leave before the required 3 years of residency?

Does the IRS force you to pay back taxes on your capital gains? If so, at what rate? Are there late fees? Criminal penalties? How does the presence of a medical emergency or other valid reason to abort act 60 influence this?

The three year rule only applies if you use the year of the move exception. Much easier to establish residency on or before Jan 1 of the first tax year you’ll file as a PR resident for.

If you did fail to meet the 3 year stay requirement, you’d want to amend all possible tax returns to reflect your new lack of BFR status in the year of the move, pay the appropriate back taxes + late fees (some % per time late),. I believe there aren’t many exceptions that aren’t already accounted for in the residency requirements (natural disaster, etc)