Wine conditioner is light in color. I personally would not use the six year old, dark, oxidized stuff. Due to the temp extremes alone I would honestly replace all additives and start fresh. Good find though!
You can easily backsweeten your wine with sugar. Just be sure that you stabilize with a combo of k-meta/campden and sorbate, after the wine has reached its natural final gravity and then backsweeten. You could pasteurize in lieu of using sorbate.