Steps to follow Import Boundaries using JOSM Create Relation for each boundary, copy details like name, ref etc to relation Split Boundary

After splitting boundaries search any closed polygon is there to cross check, if any split them. Only once we have to split closed boundary. Remaining splits at ‘T’ junctions can be under using split adjacent ways under more tools. Validate the features, this will give you overlapping features. Merge duplicate ways and keep the relations.