the safest thing to use on bare plastic would be tar wax and grease remover. It will take some time to remove depending on how cured the primer is but it should come off. Using thinners and reducers on the bare plastic will eat or melt it which is something that you dont want to happen.
Why not just sand the bumper and paint it? And yes, use urethane products whether your going base/clear or single stage so it will flex with the bumper.
Dont use enamel. It gets way to hard and youll get cracks and spider webbing if you barely tap something.