If a WO becomes commissioned and, after some time, cannot make another promotion, is there an option to go back to being a WO?

I would venture into thinking said individual did not pass WOCS (i.e. APFT, etc...) or somehow not finish WOBC. A WO1 isn't a commissioned officer until they are promoted to CW2 which is automatic after two years to the day they pin WO1.

No, as someone pointed out, you reach up or out after two looks. You can't go back to WO1, it's an appointed rank. All that you can do is resign your commission and return to you NCO rank, or join a different commissioning program that may require changing your rank.

Not sure about your scenario. However - I have seen a CPT resign his commission, become a WO and make it to CW3...then submitted a packet to return to the regular officer ranks...it was approved and he came back in as a CPT and got promoted to MAJ shortly after. Doubt that happens very often.