Yes it will get worse. Assuming you have the force touch trackpad model, which is glass, it will eventually crack your touchpad.
I have dabbled in light to medium MBP repair with batteries, screens, ports, and the 2015 era are some of the easiest ones I have done. You can do it! The ifixit articles show speaker removal, but that's not needed. Just a battery, some pry tools, some apple screwdrivers, some isopropyl alcohol (>90%), and patience. The battery cells take time to pry up. YouTube is your friend.
Batteries are $50 and up (assuming USA) on Amazon. Match your body style (ie A1398) and read reviews.
Then as for the bad one once they are swapped, Home Depot in my area takes batteries, that would be a good spot to check for disposal.
Good luck!