Do you have any electronics on your person?  I don't know many people who do not.  Turn them off and see if it has affected anything related to the fault(s).  It may seem like a long shot but it's something that must be eliminated as a possible cause.

What is that BYV26D diode called in the circuit?  CR18 or 19?
Disconnect one end of it and recheck.

Once you learn all those equations backwards and forward, you will have a much easier time with practical applications like multiple speaker cabs with speakers of various impedances.  Then you can throw power handling into the mix and find the weakest link.   

I don't think they will provide any service documentation for anything they make, even to dealers.
For that, and the SMT issue, you pretty much have to send to them or their authorized station.
But I would check with them whether you could send the board rather than the chassis.  That would save a lot of weight for shipping.

You said you heard a buzz before when powering up, so I assume you had the speaker connected.
It is best to first power-up with no speaker or load connected.
Then check that transistors are not overheating and there is no DC voltage at the output.
If all that checks out, then connect speaker and test.

Yes, I see now.
From the voltage drop (across the 2K2) you had with the 741's, assuming you are again at 22V with the 820R, you have increased from 8mA to 22mA.

So there is no defect, just an imperfect part substitution.  TL071 would have been a direct drop-in replacement with no mods.

The amp was working before with the 2K2 resistor, so changing it is just a 'band-aid' solution to some fault in the low voltage powered circuits.
You need to repair it rather than modify.
37V across 2K2 was 16mA of current, where was it going?

Does your friend have a multi-meter and know how to safely work inside of amps with power on?
If so, I would start by verifying that the preamp is getting it's proper DC voltages from the power supply.
This can be done by measuring the DC volts at both sides of the following resistors:
R20,R45,R29,R72,R10,R5

Yes, that should be correct.
It really shouldn't matter anyway, as it is a single speaker unit and you are not concerned with phasing.
The only other time it can be a problem is if the negative terminal on the speaker is connected to the speaker frame, and the frame is grounded.  (ex. some Fender Mustang amps)

As the sound came back and then went away again, it is likely an intermittent connection or bad solder joint.
Some chopsticking around the preamp area might help to isolate the problem area.

attached is a schematic that will hopefully be the correct version for your unit.
You cannot view this attachment.

First, put a patch cord between send and return, and try your guitar into the regular input again.

These amps will burn their output transistors if the heatsink is not completely mounted to the chassis, including the bar.  And the removable 'bar' has to be oriented properly or it will not mate up right with the heatsink.

I would not replace the 1C03 transistor unless it is proven to be defective.  Solid-state components do not wear out or degrade, like electrolytic capacitors eventually do, so they are not normally replaced if they are working.

One of the speaker connections will be routed through the 'phones' jack (so the amp is quiet when using headphones). 
If it is through the 'tip' connections, that would go to the speaker (+) terminal.  If it is the 'sleeve' connections that disconnect the speaker, then that would be for the speaker (-).
If you can post a picture of the trace side of the board in that area, it should be easy enough to figure out.

Here's a Carlsbro schem that uses the 1C03 in TR6 position.
Unless you are getting very strange HFE readings out of circuit, I would be hesitant to replace it just for that.
There is a 1C04 acting as it's complement in TR7 position, does yours have that?  If so, how does the HFE reading on that one compare?

Was the amp working, or did it have a fault you are trying to fix?

Nice work Phil.
Sorry to hear about your wife.  Hope you can have some peace for Christmas.