OK, I'm actually reading the manual this time (!)

Here are a couple questions:

If I want to supply phantom power, do I switch it on for both or only one channel?

The description talks about "operating in mono (L+R)" --does this refer to placing an interviewee close to the center of the pattern (and mic), or some approach to connecting the L and R channels on the camera or mixer (or not connecting one side...) or something else?

I'm looking for two additional cables--
...a shorter 5pin XLRF to dual 3pin xlr, around 18"
...a short 5pin XLRF to stereo mini plug 3.5mm. I've seen a 6 footer, but want 18" +/-

Not finding anything at Markertek... What other favorite cable sources do you know?

I have an AT825 but neither the mic nor the manual are at hand. I used to run it on battery power since the portable Sony DAT deck didn't have phantom so I don't remember but since many devices have a global phantom switch I'm sure it's fine with both connectors receiving phantom power.
It's easy to test, just remove the battery and hook it to a device that has individual phantom power switches. Listen if both capsules work with only one phantom switch on. It most likely needs both, and remember about phantom power pops when switching on.
This mic has a very wide angle of coverage in stereo, so if you want to use it for a singular subject it's best to just use one capsule positioned and pointed appropriately.
I have also used it on a static boom (in a low noise environment) pointing straight down to cover two people standing about 5 feet apart and facing each other. Worked great but of course you have to checkerboard edit the two tracks to avoid the off-mic sound of the off-talent capsule as the dialogue procedes. A good boom operator would be better.
This mic is also very susceptible to wind noise.
AT used to offer a shorter 5-pin XLR to two XLR cable with one of their stereo shotguns and this could be ordered separately.
You could also try the cable shop at Trew Audio. I believe they had shorter ones and stereo mini too.