Yes, they will share a single UV for lightmaps and there will be no overlapping as long as your original UV’s arent overlapping. When you merge those meshes it will create a new mesh in the content browser, so mesh wise you’ll have a single mesh but it will have seperate material ID’s for each mesh, so the performance may differ depending on the complexity of your meshes.
And no, they cant be unmerged, but you’ll still have the original meshes in the content browser so that shouldnt be a problem.