Just a quick one for now, since here in the U.K. it's early morning and I should be in bed!
But I do love solving mysteries like this.
Besides, the HQ-180 is a super RX.
In your opening post, picture #2, is shown a 1 kΩ resistor (R12) feeding T4.
However, looking at the cct. diag., that is incorrect. The cct. diag., shows that 1 kΩ is connected to the B+ line. It feeds connection
c on the coil (your drawing), thru the primary of T3, out on
e and thus to V3 / pin 5 (plate, 1st. conv'r.) & V17 / pin 5 (plate, IF gate). The sec'y of T3 has
d to the grid of V4 (IFA) and
a to a green wire - which is probably AGC - R13, grid leak for V4, and R17 - probably AGC decoupling.
The coil pin
b appears only to be a convenient tie-point for probably B+ (since the wire colours are red).
So as things stand, the B+ feed to V3 and V17 plates is wrong - which probably explains why the voltages you measured there were incorrect.
Al.