FieldAPI fields empty after successful entity creation via POST

Been working for nearly a full week trying pin down exactly what and where this is going wrong, with little to show for it.

There seems to be a complete lack of good examples on how to submit specific field types to a given resource, like taxonomy, tags, images and the like, so assuming you haven't screwed up the format, all we're left with is a cryptic message from the entity API (or restws) about the format being wrong, but not on what field and without a good idea of what format should work. Argh.

Anyways... My specific problem is that when using POST or PUT to a given entity (specifically commerce_product), I can successfully create the entity, but only certain fields actually populate with the accepted data. All commerce_product native fields populate just fine, but out of three taxonomy terms, only the "tags" field populates. The 3rd party "commerce_stock" field works great, but a FieldAPI string

Updating any of these fields with a PUT to the newly created entity fails silently and the entity field remains empty.

My best bet is that all FieldAPI fields added to the entity (via the UI, not via module) are the ones failing to populate.

If anything, I'm looking for tips on how to debug this. I've been in an out of most of taxonomy, field, entity and restws trying to find out where the failure lies, and have yet to find it exactly.