I'll throw my 2 cents in here to try to help too. These three switches are in same general area relative to each other (center trough, Left Out lane and Left flipper lane 1). If 76 and 77 work, I think you can rule out the green/violet wire for Column 7 unless it is not connected well to switches 75 and 78. For 75 and 78, It seems it has to be the White/Green (75) or White/Gray (78) connection at these two switches. Since all of the other Row 5 and Row 8 switches are working it has to be something with the connection of these white/ wires to these specific switches.
I have to think the same is true for switch 16 (center trough). The other switches that use Green/Brown (column 1) and white/blue (row 6) are working so it has to be the connections specifically to the center trough switch.
As for what you are seeing on the switch edges test display. The grid represents the switch matrix. Your display is showing that Column 2 Row 4 is closed or switched on/activated. In the TAF switch matrix this switch is supposed to be "Always Closed". It is also showing Column 3 Row 5 is closed. This is the lower Jet bumper switch this could just be an adjustment that needs to be made to the leaf switch to ensure it only activates when the bumper skirt is hit by the ball or pressed with your finger. the other active switch is column 8 row 4 (Thing Down Opto). I don't think you have worked the optos yet so there may be a short at this location.
If you notice there is a column of dots to the left and right of the 8X8 grid. These represent the Direct connection switches (D). These are coin switches test/menu switches and flipper switches (buttons and End Of Stroke EOS).
DumbAss may be the smartest person I've ever consulted on this stuff. He always has good advice and a wealth of pinball/electronics knowledge.
Good luck and keep it up. You will get there sooner or later.