Tim, that knocker in your first picture unplugs to make removal of the bottom board easier. Someone may have just forgot to plug it back in when they worked on it.
One word of advice: Get everything working. From start up, to reset, to game over before messing with trying to improve play of the pop bumpers etc. That way, you start from a fully working machine and if you mess something up, you'll know it was probably the last thing you did.
As far as your pop bumpers and slingshots, there's a good chance that it always played that way. Early 60's Gottlieb games didn't have to power of later DC bumpers or modern games. Cleaning the path and changing sleeves in the coils may help a little but you'l probably still be disappointed. You might want to try high tapping.