The key should open the door even if the cars battery is removed and can't see how changing the keys battery would stop you doing that :wacko: .
Have you tried disconnecting the cars battery for a while and reconnecting it to see if it resets?
If the piston won't go in then they won't be able to fit the new pads to it so if they fit new pads to the other working side then you'll get uneven braking on the rears and any problems with braking is not something I'd be happy with!
If you do want a swaged end then try this http://pages.ebay.co...3115021&alt=web as I only got one from them a few weeks back and although out of stock maybe able to get you one :) .