The only problem I can see is that the battery cover is stuck. Is there a specialised tool (ie. one that is not a coin) that can get these off? From reading around it sounds like a PX400S battery would be a good replacement battery, if I can get the cover off.
A routine problem. Sometimes the only solution is to drill out the cover and replace. Having been faced fairly often with this problem, we made our own tool. A large screwdriver. We ground the head of the screwdriver to match the curve of the Pentax slot. Then we could apply plenty of push (to preclude buggering the slot) and plenty of turn at the same time.