The battery is soldered in. I have never had to change a battery but the service manual has a warning that if it is disconnected the board needs to be sent back to the factory! Keith may know more but my advice is if it ain't broke then don't fix it!!!!
The mute relay should be soldered with the minimum of heat as with all pcb type devices it is possible to melt the internal solders.
Good luck with the stringing. If you find a knack that makes the job easy then please let me know!