If you were ineligible for the Roth IRA contribution only because your own MAGI filing separately was above $153,000 but filing jointly your MAGI together will be below $218,000, yes, amending to joint filing would eliminate the excess contribution. Yes, you are permitted to amend to change from the separate tax returns to a joint tax return. The instructions for Form 1040-X have instructions for amending from separate to joint.
