My known good bobbins measure 1.296", end to end. They fit tightly into the low bobbin winder and the machine sews with them.
I looked at the bobbin winder while I was back in there. One end or the other MUST screw on. If all else fails, I'd be inclined to pad both ends with some leather, clamp them in vice grips and ... gently ... try to unscrew, bearing in mind that they could be left hand threads.
In rehabbing guns, many people swear by a 50/50 mixture of acetone and automatic transmission fluid and soaking for a day to weeks to break frozen metal things apart. It needs to be in a sealed container because the acetone evaporates. I bet it'll eat every trace of paint.
There's a variant called "Ed's Red" which adds a 3rd ingredient. If interested, an internet search should turn the formula up.