I'm no ford expert but I beleave if you pull the starter out you'll see the drive on the end it should be extended fulll out if it is in it will be jamed to the ring gear and won't turn which would explain the burnt cable. My 8N tractor did this when I had a low battery it just got stuck I removed it freed it up and put it back in. By the way you may have burnt up the solinoid but I jump my 6V 8N all the time with 12V and in 10 years it hasn't hurt it. Lucky I guess??
One other thing you could try is rocking it while in gear BUT with the ignition OFF.