"The instructions say that IRA contributions have to be added back"
No, not for Medicare IRMAA. MAGI for Medicare IRMAA is AGI plus tax-exempt interest (Form 1040 line 11 plus line 2a).
(You appear to be looking at the calculation of MAGI for the purpose of deducting an IRA contribution or for making a Roth IRA contribution, which are an entirely different MAGIs.)
