This one quite a complicated request.
Requires you to edit clif_parse_LoadEndAck to auto unequip the left hand when loading map, but not when is a shield.
Then edit pc_equipitem to disallow equipping the left hand if not shield.
How to add custom skill on 3rd job tab?
I tried adding it to data\luafiles514\lua files\skillinfoz skillinfoz\skilltreeview.lub but it always shows on 2nd job tab.