Seen single boundary member line sections used in a dozen boundary relations i.e. draw once, split where smaller areas start and create boundary relations for those. ( I only do this for boundaries and do not make them share landuse/landcover outlines as when those change you’d be moving boundaries too.)
No, when cutting up the boundary relations members are transparently split, just get more members. Best to do this with JOSM and the relation toolbox plugin attached.
On 1) Importing is always a touchy subject as it has to be properly licensed.