I'll try and answer in order.
1.Your original special solenoid connector should have the 2 orange flipper wires at the bottom of the connector, pins 1 & 2 on the driver board and the other wires that are blue /? follow upwards with the keying pin at pin 5.
as you know the headers are reversed and you have the flipper wires at the top of the fuse saver board. This must be fixed.
what might be easier is slicing of both the locking tabs and just turning the connectors around the way they should be?
you can the end of 'your made loom' alone at the driver board but must alter the other side to match where the original connects to the fuse saver board, with the locking tab gone just switch it around, then check continuity, visually should be fine as your wires are different colours.
otherwise you need to remove the wires from the connectors as discussed in post #151.
2.cool
3.you can't test a diode in a circuit, this is correct, you need to remove 1 side from the circuit.
but you can check the coil resistance with a diode attached and either wire removed from the coil to break the circuit.
4.to test the resistance of a coil use your lowest resistance setting and a wire removed, and when checking a flipper coil remember to put a piece of paper or
similar to break the EOS contact.
5.i suggest testing the flipper coil resistances, both windings as per: https://www.flippers.com/coil-resistance.html
don't forget to break the circuit of the EOS contacts.
say you thick winding is 4ohm and your thin winding is 330ohms then across both you should get the sum of the 2 windings, that being 334ohms.
6.i suggest replacing the coil diodes on all coils. 1N4004/1N4007. This may sound extreme or not something you want to take on, but it's what I do for peace of mind. As you'll be testing diodes, since one leg is already removed, maybe remove the other and replace it. It's a slow job and instead of removing the original diode by unwinding it from the solder tab/leg, you could simply cut it off. Unwinding them is quite time consuming and risks breaking the fine coil winding wire if all the original solder isn't removed.
a shorted diode will cause a fuse to blow because it's giving the current a direct path to ground.