Sorry to be so slow in replying, I have been away and looking at schematic diagrams on the phone is no fun!
I quite agree with your comments about difficulty in distinguishing an open circuit transistor from one which is biassed to cut off. With the power off you can do diode checks on b-e and b-c junctions but it is not very convenient unless the transistors are socketed.
Your measurement of -0.84V on the base of U973C is very puzzling and may indicate where the fault lies. U973D and U973E form a differential amplifier with a constant current source in the common emitter supply. Whichever of the pair has the more positive base voltage should take the majority of the current. When the regulator is working properly the collector of U973E (base of U973C) should sit at +5V plus the forward base-emiiter voltage drops of U973C, Q985 ansd Q988 so about +7V. The voltage divider of R970, R971 sets the base of U973E around 0V, the base of U973D is grounded via R975 and the gain of the differential amplifier stabilises the output of the regulator at +5V.
You measure -0.7V at the output of the +5V regulator so R970/R971 will set the base of U973E around -5V while the base of U973D is 0V minus the voltage drop of its base current across R971. The base current shouldn't be more than tens of uA (the emitter current is about 0.5mA) so the base of U973D should be only fractions of a Volt negative. In this state U973D should be drawing most of the common emitter current and the collector of U973E should be at a much higher voltage than you measure. Probing U973D and U973E might confirm this or we need to look elsewhere.
If the measurements show that U973 is suspect you can replace with a CA3046 or get the Tektronix replacement - is it socketed?
PS There are some professional engineers around this group who can be much more precise and more authoritative than me.