Unless it’s an entirely different map you can’t bake that anyway then.
Generally, yes. It takes a lot of lightmap data and a huge amount of RAM on your part to even bake when you have a lot of vegetation or open world sized maps, not to mention the sheer filesize of the maps. If you have a ton of grass or trees, just set those individual foliage types to movable.
Duplicate the map first then.
Yes, just only set the sun to movable and nothing else then.