The background layer (what you normally start out with with just a photo) is always locked and that doesn't prevent editing. However, the amount of 16 bit editing you can ever do in PSE is somewhat limited, only basic edits, nothing with layers or the artistic filters, for example.
With image in 8-bit you would be able to access 100% of PSE but with 16 bit the editing is limited and also usage of layers.
Found one nice discussion here on the 16 bit support/difference from PSE perspective. Should be helpful