Nice work getting used to a nice spectrum analyzer. And doing some calculations. Keep in mind the bandwidth at the receive output is about 3 kHz.
I suggest starting near the back of receiver and going forward. Turn the volume all the way down, place a signal within the IF of the 11ish MHz xtal filter, both before and after. BTW if you have a leftover xtal, easy to make a generator that outputs approx s9 and s3 signal levels, I homebrewed one from norcal qrp website. Or use whatever signal generator thats there, but highly attenuated. if that seems good, work your way toward front. Not a bad idea to check voltage bias of each Rx path amplifier.
Solder connections can look good and be bad.
Thinking ahead, for transmit you should find or make a dummy load. 4 2 watt 200 ohm resistors in parallel etc.
All those mixers have transformers, always possible the insulation isn't off a wire. When you check continuity, never touch wire but a connected pad is better than edge of solder.
I presume you checked frequency of each oscillator and its presence.