I estimate my income next year will be very low, so I need a premium tax credit and I need to have it 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 enrolled in Medicaid, will I have to pay back the premium subsidy?

No, your final premium 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 pay back the premium tax credit you received. In fact, your final credit amount will likely be larger than the amount you received in advance. This is true even if you live in a state where you are not eligible for Medicaid due to your low income.