if you remove this I would put some bracing higher up in the roof with plywood triangles screwed to joists use 3/4 ply build up in pieces that you can get through loft access bring them down to a height that will clear head high
if you are laying hardboard it needs to be soaked overnight laid flat do not lay tight up to each other leave a good 3/16 gap at the joints otherwise it will ripple also lay smooth side down
this may be easier from below but you would need to jack up floor and bolt extra timbers to each side of existing joists and if possible put in extra joist as they are wide spacing on existing joists