Hey Flip,
You've cleaned up your DS-5 nicely but I'm not sure I've led you down the right electrical pathway... (Remember, I don't really know what I'm talking about!)
To correctly operate in mono there should be THREE 20k resistors involved. (Is there one more that we can't see wired into the jack/switch?) You want one from your new input, one from pin 2 of the 5-pin connector and one from pin 3. The other end of these resistors should then meet at the mono output jack.
In other words, as long as all three preamp outputs (your Series has two outputs, your Orion has one) are 20k away from your summed mono output, then they should all be happy. If you shut off the Series bass, which essentially shorts the outputs, the Orion bass shouldn't be effected.
If there are only 2 resistors in your circuit I would think one of your Series outputs would appear suddenly louder than the other.
Furthermore, I'm not sure this wiring is going to work in stereo. Although if I'm reading correctly, you only ever want the Orion to appear on that bass output jack, even if the Series is running in stereo? If that's case you've almost got it, but may still need one more resistor to make the levels all match.
If you wanted your newly added inputs to appear at both stereo outputs, that's a bit trickier. I understand the concept of summing signals to mono - as I've been trying to describe - but splitting a mono signal to two outputs is a bit more complicated as far as resistor values, loading, etc... I'd need to do some experimenting to see what worked.
At one time I had a rack piece I called the sum-thing which only did resistive summing. Signals A+B from the bass were summed with the output of a stereo chorus (effect only when turned on). Those two outputs drove the stereo power amp and speaker rig, and they were summed again to make a mono signal for the DI out. Basically 4 to 2 to 1. And to my surprise, it worked. Ha!
You're wireless rig's output is also an active signal so that can be summed in the same way you're thinking, at least to mono... Just watch out for added noise.
Hope I didn't make you do any unnecessary soldering!
Jimmy J