I've made some headway on understanding this issue.
As to the question of whether I made enough profit in 2025, that certainly could have an impact on the calculations. In this TurboTax article, I learned that the health insurance premium deduction can't exceed the earned income you collect from your business. So I'll add that to the disclaimer that these calculations (the one for medical expenses as well as for self-employed health insurance) may be purely academic when it comes to federal taxes.
But that doesn't explain why the numbers don't add up, and an answer to that question is a bit more difficult to find. Based on what I read in this community discussion, I have gathered that there's a sort of circular reference happening (the amount of the deduction is based on the amount of the premium tax credit, and the amount of the credit is based on the amount of the deduction), which requires an iterative calculation to solve. That seems to have been a problem for TurboTax in the past, but it may be resolved now. The takeaway is that the amount on the Medical Expenses line is not a simple sum of my health/dental claims and the premiums I paid for my Marketplace plan (1095-A column A minus column C). My MAGI needs to be calculated before the self-employed health insurance deduction, and then it can be used to calculate the deduction based on the amount of subsidy I should have received.
The question that remains is what calculation should be used for Missouri, and I remain in the dark on that. I have checked a competitor's online tax preparation site, and it doesn't allow any of my premiums to be deducted. I will post here if I figure that out... hopefully this helps anyone who has similar questions!
