Are all PP(Polypropylene), i.e. resin code 5 plastics, safe for microwave and steaming?
No.

Just because the polymer is of one or the other kind does not let you say what industrial process actually synthesized it. Subsequently it is impossible to know what trace chemicals are left in the polymer packaging.

If if isn't labelled as microwave-safe, it isn't!

They test the package for it's microwave compatibility, not one type of polymer each! It seems that resin code 5 polymers are often used for microwave-proof packaging, but again: You don't know how it was made, you have no idea what's in there, don't microwave it (together with food or something else that you then ingest).