Looks like you have done all you can for the display, just break down and order another one. If you have any interest in upgrading to 7 digit displays I know Jim from Boston Pinball has made a 7-Digit display that plugs into the standard harness for the 6-digit and then Pascal has likely already got the code to work with them. Then you could get LED 7-digit displays without any wiring upgrades. I have yet to order these as I sunk a fair bit into my HH already but I will eventually do it.
Ok onto the coil issue. First thing to check is that you have 24VDC on the Red/Yellow/Yellow wire of the coil. I'm assuming you do as it sounds like the trap door and up kicker are working, they all share Fuse F14 on the main PF. Given you have a brand new Pascal board that leaves the Q1 2N5875 transistor mounted on the bottom of the main PF right beside the coil as the main culprit. For many of the System 80 Games they used under PF mounted transistors for the high current stuff like kicker coils so they could use low current transistors on the driver board that are normally used for controlled lighting to run much larger loads like coils.
Transistor and Coil1.png
In the image above you can see the transistor and coil in question. The Yellow/Purple/Purple wire runs directly from the coil to the Transistor beside it. The transistors job is to close the connection between ground and the Yellow/Purple/Purple wire when it receives the control signal on the Brown/Orange/Orange wire.
Transistor and Coil2.png
Before assuming the Transistor is bad we should check the green wire connecting to its case is in fact a solid connection to ground using your multi meter. If that tests good then you should follow this excerpt from Clays guides to confirm the transistor is bad.
It is best to isolate the 2N5875 from the driver board. This can be done easily by removing the connectors from the driver board to the playfield (or remove the one lead from the transistor that connects to the driver board, see two steps below). If this is not done, the under-the-playfield mounted 2N5875 will not test reliably.
Put the red lead on the metal case of the transistor, and put the black lead on each leg one at a time. If the transistor is installed in the game, a reading of .5 for each leg should be seen. If the transistor is not installed, .5 for one leg, and nothing for the other should be seen The values can be from .4 to .6; anything else and the transistor is bad.
It's always best to check the wiring on the playfield mounted transistors too. I've seen them mis-installed by previous repair people. With the transistor front facing left, pins right, long part of transistor up, the farthest pin from you (base, white/red/red wire with pull up resistor) is always connected to the driver board. The nearest pin to you (emitter) connects to the NON-banded diode side of the coil. The case (collector) gets the green ground.
Important Note: if the pre-driver MPS-U45 transistor on the driver board for the 2N5875 is bad, the 2N5875 could test as bad (even though it is not)! Again if the under-the-playfield 2N5875 is isolated from the driver board, this will not be an issue.
Now put the black lead of the DMM on the BASE of the playfield mounted transistor (this is the transistor lead with TWO wires connected). Put the red lead on either the metal transistor case (collector), or the emitter (the other leg). A reading of .4 to .6 should be seen. Change the red lead to the other transistor terminal, and again .4 to .6 should be seen.
I suspect you will confirm the state of the 2N5875 is bad, PB Resource sells them for $5.10 each and its an easy replacement.