True free play isn’t really an option on an OEM board. You can set it up to award a free game at a low score so you achieve it every time you play. That way there is always credits on the machine. As for the battery, remove it and install a memory capacitor. It is the easiest and cheapest fix. NVRAM is over kill on Gottliebs.

You’re battery might have been removed already. The OEM battery was a large black rectangle. Install this in place of it and you are good to go. Just need to solder it on. Easy to do.

ebay.com link: Electric Double Layer Memory Back up Capacitor 1F 5 5VDC 2 pcs

It is the orange thing on the largest board. Can you solder?

somebody added the orange batteries to the board. They have failed or weren’t installed correctly. Remove them with a soldering iron and install the memory capacitor the same way. Pos to pos and Neg to neg.

You need to revisit the switch because odds are that is your problem. With the glass out, start a game and smack the playfield with your hand in different areas. Also try manually firing the other pop bumpers. My guess is that action will fire your questionable pop bumper. After you verify that slide in a piece of paper (like a business card) between the contacts of your questionable pop bumper and repeat the test. My guess is that your pop bumper won't be firing anymore. That will prove if it is the switch or not.