Seems may be a couple of issues.

I would..
Reseat all the ribbon cables on both ends.
Press on all the IC socketed parts on the MPU board.
Diode test on all these on the driver board. Make note of any that are lower than .28v

If all the diodes test close to one another, not below .3v, then see if the above helped.

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Even with the coin door closed?
Is the flipper still up?

Did you also diode test the diodes to the left of that blown fuse? I forgot those were under that ribbon cable.

With power off can you test these transistor to ground with the ohm setting to see if you have any shorted? This is likely one shorted causing the fuse to blow. You'll know if you see one less than 50 ohms and the others are much higher resistance.
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Press select to be sure to take off that continuity beep. Just the horseshoe symbol for ohms should be displayed. Touch the meter leads together to be sure they read less than 1 ohm. If so, then it is set up correctly.

Do you know if the previous owner mentioned anything about hearing a coil lock on by chance?

The opto board would affect switches. Though, if the optos can't be seen, the game will think there is so ball stuck when you turn on the game and do a ball search, but will not fire all coils at the same time.

What coils are not working? Not sure whether the same fuse number on the Rottendog driver board matches up with the original WPC driver board.

The DMD is still not displaying properly?

Are there any unlit LEDs on the driver board with the coin door closed? 2 are out on the picture and I wanted to be sure.

So it seems something is loading down that 50v. Can you lift the playfield and be sure you can easily move every coil plunger? Something must be drawing a lot of current and that blew the fuse in the past.

I agree that it seems very likely to be a transistor issue with it. Rottendog does not use TIP on their driver board.

OP, can you inspect the right flipper? Broken diode, shorted diode?

Can you check the resistance on both windings? One coil winding is attached to the EOS switch, so you would have to open the EOS switch to get a good resistance measurement on the coil winding.

Yes, that is about right on both windings. For that coil number 4 ohms is for the high power flip and 126 ohms is for the hold section.

Can you disconnect J133 and J134 from the top of the driver board to see if it it will boot up and display ok on the DMD?