Usually you would avoid going back. Your issue still looks like an issue with the rest pose.
Default MH 1.1 (game skeleton) export (unmodified restpose):
Modified rest pose of MH 1.1 (game skeleton) export:
Modifications (not perfect but already a lot better …so probably a starting point) - Note this is based on the default game skeleton export (not the UE4 Tool):
Rotations:
upperarm_l and upperarm_r x 6
lowerarm_l and lowerarm_r x 5
lowerarm_r yy 20
lowerarm_r y 17
lowerarm_l yy -20
lowerarm_l y -17
hand_r yy -20
hand_l yy 20
hand_r xx -7
hand_l xx -7
hand_r y -10
hand_l y 10
index_01_r: zz 10
index_01_r: yy 5
index_01_l: zz -10
index_01_l: yy -5
ring_01_r: yy 10
ring_01_l: yy -10
Afterwards applied pose as rest pose and changed bone-roll to Global +Y axis… added quick some twist bone (even not perfect but already a lot better).