Everything looks fine to me.
Yup, installing them upside-down helps clear that up easily.
Looking at my schematic yesterday it seems that one side is not conducting. You do have voltage at the plate of the driver tube so the circuit is clear to that point. Usually it means that the cathode circuit is open somehow. Try this, set your meter either to the lowest resistance setting or the diode setting.
This will test the diodes. Measure resistance from the pin on the tube socket to terminal 3. This doesn't touch the diode leads and proves the solder joints are good.
Measure one diode from tube pin 3 to T3, then swap the leads and measure.
Measure the other diode from tube pin 8 to T3, then swap the leads and measure.
In both cases above you should get a high resistance measurement one way and a low one the other way. Since you have two diodes right there it will be easy to compare one channel to the other.
Edit:If you find one or both LEDs open, infinite resistance, both ways retouch the solder joints. By that I mean wet the soldering iron, touch it to the tube pin or center pin or terminal at ground (T3?) and when you see it become liquid count 1..2 and remove the soldering iron. So just liquid and the heat off. Too much heat can fry the LED.