Am I eligible for another Overseas Service Ribbon if my tour is shortened to attend Warrant Officer Candidate School?

yes but, the reg can be interpreted in this case to or not to support.
So if you want to wear that ribbon, I would write an email to HRC, lay out the events, and get their opinion.
If HRC (or who ever they pass you off to) supports wearing the OSR, then you take that letter and it goes in your "I love me" folder along with every other order, award, and document you have. To be used the day someone questions your wearing of the OSR.

Same drill for me. You are being reassigned. You completed your tour and PCS to WOCS. You did not request a curtailment or opt out of your overseas assignment as previously suggested. Now, you need to speak to the tech at your servicing S1 or the S1. They interpret regulations for a living, specifically in the area you are wanting help with. RP is great but nobody here makes the entry on your WEB.

Based solely on what you've provided, I can see where someone might reasonably interpret that your submission of a WOCS packet as being your request to leave (to attend that school). However, you could easily (and reasonably) counter-argue that your leaving to attend WOCS is for the "convenience of the Government". After all, they have a mission that needs to be fulfilled (creating more Warrant Officers), and they have selected you to help fulfill that mission. So, assuming no other interpretation from someone with more relevant knowledge of the current regulations (I retired in 2011), I'd say "yes".