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since Croatia is not officially in the Schengen Area, would it satisfy the document requirement (where I must have a return flight from the Schengen Area)
You should buy a train ticket from Slovenia to Croatia online and attach a copy if you can. If you can't, explain that in a cover letter and outline your itinerary.
Anyway, there's no guarantee you'll get a double/multiple-entry visa. If you don't, you'll need a Croatian visa as well.