Move it to another room ASAP then sterilise anything that has come into contact with it. It doesn't really sound like you have vinegar, just young cider, especially if you're trying it as soon as the krausen had fallen. It's not unusual for ciders to ferment lower than 0.998. I don't know the answers for your other questions but good luck!
Primary: Cherry Melomel
Bottled: JAOM, Amber Ale