– in the Skateball switch matrix schematic (which is available on the ipdb), it DOES show high voltage (500V) caps across the EOS switches, but this is referenced in note 5 at the bottom of the schematic to be used for Germany.  My skateball does not have caps across the EOS switches, and I am curious why only for Germany was this recommended? 

I'd guess tighter EMS noise specification / requirements in Germany is the reason for the cap across the EOS.

I'd replace the switch matrix diodes and use 1N4004

Are there time delay filter caps on the switches? If so have a look at them, just remove to test - you'll miss some scoring on that switch (they just make it look as though the switch is closed longer than it actually is) but it will remove the false triggering if that is  the issue.

I'd also have a look around the area where the solenoids are fired. I'm not familiar with your game to know which boards used, but I have mid 80's Bally that uses a BCD to 10 way chip to decode and select which solenoid to fire. I suspect mine might occasionally "monkeys around" in the way it fires.