Just thought I'd follow up with this result:
That rice wine that had the tiny bubbles after shaking it vigorously...well, it was fizzy. I decanted, sloshing it about quite a bit and let it sit for a couple of hours. I tasted, it was better. Then, I thought, what the heck, even though it "nott-a-sake", why not treat it like sake and warm it up. What do I have to lose?
Son of a gun, it was great! My husband suddenly fell in love again...with my wine. It really did taste like a nice sake.
I think that proves the hypothesis...it's the "dreaded CO2". Now to learn better degassing techniques....
Thank you all for your input. I'm going to have a warm sip right now.
