In the absence of detailed information, a ballpark estimate would be like:
50 m2 gives around 30 m of perimeter x say 0.5 m thick = 15 m2 of wall.
50 m2 x 160 kg/m2 = 8000 kg live snow load / 15 m2 = 533 kg/m2 (around 100 psf). Roughly equivalent to having people standing on top of the wall crowded as closely as possible.
With the dead load of walls and roof already compressing the bales, there might not be much further movement from seasonal snow load. I expect the bales could structurally hold this fine, but there might be
enough movement to crack plaster... an experienced strawbale builder would need to weigh in on that.
There is no particular reason to have a flattish roof to collect rainwater. Gutters can do that regardless of the slope.