I doubt that TBH, if your confident that it's been getting a good +12V from the other battery (which I'm sure it was) my money would be on the solenoid part of the starter, is there anyway you can give it a knock and at the same time apply a good charge to it, that should help to free it off. I realise you've tried this but there can't be much more of an explanation apart from something more serious like a seized or locked engine.