You're in luck. I have been working on getting my Caveman up and running the past few weeks, and I think I can help you.

First of all, like ForceFlow pointed out, it is best to make changes to a game one at a time to make sure you don't mess something up between steps.

As for your boot problem, take a look at this post: https://pinside.com/pinball/forum/topic/caveman-no-pop-bumpers-are-working-where-to-start#post-4504524

The MPU in caveman won't complete it's boot cycle if it cannot communicate with the video card. The default language for the video board is German, and if the video board is not receiving input from the MPU via the lamp drivers, it will display German. So I would start by double checking the connection work you did at the MPU related to the lamp outputs. If those look fine, I would trace the lamp output wires from the mpu to the video card.

It sounds like your MPU is still not booting. It’s common for system 80 machines to momentarily pull in their coils on boot (sometimes called the system 80 ‘clunk’). I’d try to get it to cleanly boot before worrying about drive transistors. If it’s constantly pulling in, pull the coil fuses (they are under the playfield). Don’t worry about the hum, I noticed that of all my system 80s, caveman hums the worst.

I’d start going through the MPU boot issues of Pinwiki to eliminate issues with your MPU. I can give you a trick to test if your reset section is working or not. Pin 40 of the main 6502 processor is the reset line. This is supposed to be held low for about a half second when the power comes on then it goes high and the MPU starts executing its program. If the MPU starts too soon because of a bad reset section it won’t boot. You can force the MPU to restart by taking a jumper wire tied to the ground reference and touch it very briefly to pin 40 of the 6502. If the game then boots up you have a bad reset section.

I don't think the ground mod is causing your issue. It's more likely that you have some issue with the re-pinning you did. Something is not making a connection to the video box causing the boot cycle to not complete.

Moving old games always runs the risk of something not working when you set it up again. I have moved more than one game only to have something shift during the move that caused it to not work at the new location. This weekend I am going to be moving all my games to a new house, and its almost certain at least one will not work after they are setup in their new location.