I estimate my income next year will be very low, so I need my premium tax credit all paid in advance. If, by the end of the year, it turns out my annual income was even lower than I expected and I could have qualified for Medicaid, will I have to repay the advance premium tax credit paid on my behalf?

No, your final premium tax credit amount will be determined based on your income for the year as reported on your tax return. The fact that it ended up being less than you expected does not mean you have to repay the advance premium tax credit paid on your behalf, even if you could have qualified for Medicaid. In fact, your final credit amount will likely be larger than the amount you received in advance. (This is true even for consumers who live in a state where their income is not low enough to qualify for Medicaid.)