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".

You have successfully begun your transition to the WO corps. “This” is definitely the priority for you as you embark on the next phase of your career.

Sarcasm font....

I have gone through 614-30 and have to agree with you, so long as your personnel center (S-1) also agrees and awards the OSR based on the reg.

The fact that your tour was shortened by DA should qualify you for the OSR.

the only sticky thing I uncovered is Ch. 7-2 where it says that a normal tour is 36 months:

"7–2. Tour lengths and exceptions
a. Unless otherwise indicated in the JTR, Soldiers assigned to Alaska and Hawaii will serve a minimum tour of 36 months whether accompanied or unaccompanied."

How about making these available to those of us who did extensive overseas tours prior to 1981.

It is convenience of the Government. They could have assigned you a WOCS date for after your tour completion.

Wrong! Read the regulation. You completed a tour. You left early at the convenience of the Government.

I would argue that your submission of a packet for Warrant Officer would be a request to DEROS early. The reason why is because, even though selection to attend WOBC can be interpreted as "for the convenience of the government," in submitting the packet, you knew the timelines involved. Even if you didn't know the start date of the school at the time of submission, you knew that if you were selected, you would be slotted for the next available class after the board. So, at minimum, that would mean that when you submitted the packet, you knew that there was at least a possibility of early DEROS as a result of submitting the packet. Not that I'm trying to come off harsh or disrespectful, but rather just offering my understanding of the situation.

Because it's all about that salad bar.