Sitting and Performing Salaah

Sitting and performing Salaah in a coal mine

Q: I work on a coal mine, where the roof is not high enough to allow one to stand up straight. We work morning shift for a week and night shift the next week. Dhuhr is delayed until returning to surface when working morning shift, however salaah is performed underground when working afternoon shift. As one is required to stand upright, is it required to a) sit and pray (and then repeat the parayer when on surface) or b) stand and pray (even though one stands slightly crouched)?

Sitting and performing Fardh and Waajib Salaah without a valid excuse

Q: If a person prays a fardh or waajib  prayer sitting down, without a valid excuse, is his prayer considered invalid? I recently heard from a Mufti that the prayer itself would be valid, since he did not miss a fardh element of prayer by sitting down (and not standing). What is the position of the Hanafi madhab in this regard?