The part numbers 79M18A denotes that this component is not a transistor but a 3-terminal voltage regulator IC where "79" would make it a -V regulator and "18" for the output voltage hence, a 79M18A is a regulator for -18V. Conversely, 78M18A would then be +18V regulator.
Thus for the 7805, "78" would make it a +V regulator and for the "05", a regulator for +5V.
These regulators do not test like the transistors. Other than a (near) short, resistance check serves a very limited test. The prescribe test is active; i.e. supply these regulators with a higher source voltage and measure the output voltage.
Based on field experience, the 78xx regulators do not fail that often compared to the 79XX regulators. In some other instances, the input and output legs of these regulators (78xx & 79xx) may develop cold-solder over time.
Another culprit associated with regulators failure would be the electrolytic capacitors at their input and output legs.
Was the A1726 checked out-of-circuit? Again, am concerned about the 0.02 reading both ways of the Base-to-Emitter
The driver transistors would be those closer to the white resistor modules. 6 of them would have their own small aluminum heatsink per pair and the other 8 with none.
The smaller transistors encircled in red serves as thermal sensors and alters the BIAS accordingly (inversely). The higher the temperature of the big aluminum heatsink, the lower the BIAS voltage / idling current. This is meant to prevent thermal runaway of the big output transistors. These thermal sensor transistors rarely go bad.