Some basics about board repair.
Boards have chips, other components, and connectors. Everything on the board is there for a reason, and everything needs to be connected by tiny little copper 'traces' that are covered by green conformal coating.
Sometimes the copper traces get broken. Sometimes the 'pad' where a component is soldered to doesn't make good connection to the trace. Sometimes the solder is cracked, or just poorly done and doesn't make connection.
So technicians install 'bug wires' or 'jumpers' to put a piece of wire that connects something that needs to be connected. This is done in addition to the traces that should be doing the same thing, but for some reason aren't.
Specifically, the jumper you have on your board is connecting switch row 2 to the component D4. At some point, something wasn't making connection, the technician put an additional wire to make sure it was connected. This work was probably done at the same time that they replaced U18.
Now, add battery acid (alkaline). This stuff eats away at the traces, at the solder connection, and gets inside the components themselves. It gets on the metal of the pins of the connectors, it gets inside the connectors, and it gets inside the wire. It eats away at the metal, and makes things not make connection.
The first instinct would be Switch Line 2 isn't making connection to D4. Just put a jumper to make sure that is making connection. And if battery acid (alkaline) weren't involved that would be a long lasting repair if you do proper soldering.
But with battery acid (alkaline) it's still a repair... but the acid is present in other places, and going to be causing you problems eventually everywhere there is enough acid (alkaline) to make something not make connection. So fixing a connection to make sure that pin 2 on J208 and J209 is properly connected is great... but if all the pins on all six of the bottom connections are grey, greyish-green or have white powder on them, you've got fifty other pins with creeping problems that might show up later.
My friend Lenny says 'Replace all the grey stuff with shiny stuff', and yeah, you can do that. You can use a mild acid to counteract the alkaline (a lot of people try vinegar), you can replace all the diodes and resistors and chips that have tarnished pins. It's a lot of work, but you can get years more use out of your board, or maybe your board will work nearly forever after you've done the acid (alkaline) remediation.
Board repair guys do this.
Depending on the state of the board, I've bought replacement CPU boards for six out of the last eight battery acid (alkaline) compromised boards I've had go through my shop. Even after replacing the board, the wired plug connectors on the bottom need to be replaced if they have grey, grey-ish green, or white powder on them. Battery acid (alkaline) is insidious. It creeps through the wires to cause you problems later.
Your board is compromised. Getting the board fixed or replaced is your next step towards a working pinball.
Good luck!