Married Filing Separately

Hello, I have a few questions surrounding Married Filing Separately:

 

1. My husband and I have always filed separately due to him having tax debt he came into the marriage with. What happens if he passes away, is his tax debt my responsibility?

2. This year I accrued a lot of out of pocket medical expenses i.e. IVF, how does writing that off work? Can you use standard deduction to do so? Expenses are about 35% of my annual income

3. Does filing separately affect me being able to open/contribute to a Roth IRA? I'm at 90k, husband 290k