Already done.
Yep, they're only set to help with roll.
By horizontal stabilizer, do you simply mean another wing segment? Or a wing segment with ailerons on it? Or canards? Etc. I'll be trying a wing segment with ailerons on it while I await your response.
Will be done, and I'll edit this section with the results (for moving my CoL as well).
Results at the bottom
Actually, I was entirely unaware of that. Thanks for the tip!
EDIT: OVERALL RESULTS:
Holy crud!
I did everything that was suggested (CoM further foward, CoL behind CoM, add rear stabilizers, etc.) and took it out for a flight. I couldn't get it to flip around, and I tried as hard as I could! Thanks so much for the help!