If a plastic is marked PP as in Polypropylene (i.e. resin code 5), is it automatically safe for steaming and microwaving, such that it wouldn't leach chemicals into the food(besides not melting)?
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).
Consumer polyolefin plastics contain trace mold releases that adversely affect biological studies. For human food use, one presumes Generally Recognized as Safe as long as the contents stay water wet, below 100 C. Burning on residues (cheese) allows the temperature to skyrocket.
Polyoelfin containers are penetrated by hot lipid and especially hydrocarbons (red lycopene in tomato products). Do NOT use dishwasher "bleaches" to remove the internal stain. The active ingredient is dispersed dibenzoyl peroxide (Bondo cure). Sure, it diffuses in and free-radical breaks up the molecule's conjugation, rendering then colorless. It also free-radical scissions all plastics in the dishwasher. Things go brittle and fail.
Basically any plastic used in a microwave is problematic. I have seen several containers at work marked "microwave safe" that clearly reacted into the food when used in the microwave.