Thanks!

It will record separate channels for 3/4.

Be careful of the SD card media. Depending on the firmware version, there were some issues with loss of data if not using an approved SD card.

Also, on my 60D, the input on 3/4 runs very hot so I need an attenuator if I am taking a board feed into 3/4.

Yesterday I recorded a show in M/S mode (located on the FOH, attached to camcorder's shoe mount) but still can't figure it out how to decode it properly....

I've tried everything I can think of to make that work using an array of effects related to MS etc, and failed.  You pictured MSED settings are correct.  But there's something amiss with the original recording.  Step one, IMHO, is to set up the device at home, record in MS, and simply walk round it centre, left and right, and for that matter, back.  Then see whether that decodes correctly (in other words, you should hear the correct placement).  If that also doesn't work, there would appear to be something wrong with it.

Finally - after hours of checking YouTube reviews - me obsessive?? - I have found one which tests the stereo audio, standing on bridge across a waterfall.  Ok, nothing to do with music but at least this shows it does work and the stereo image is satisfactorily wide.

https://youtu.be/M5SM-0xFiik?si=JSo6ClB0bLP2seXh&t=524

The link should take you to 8:45 in the video where the stereo test begins (albeit initially in mono).

As for comparison with the Zoom Instamic device, I am not in a position to compare them, apart from noting that the price of the Insta360 device seems to be half the price of the Zoom device.  Maybe it's half the quality - I don't know.  One thing that seems obvious is that the three mics on the Insta360 device appear to have the potential to act as a mid/side mic, and I wonder whether down the track they might add M/S options in the menu.  But of course M/S processing of an A/B file can be done in post-production anyway.

Thanks for that.  The two photos I posted above are the Cable Techniques plugs you mention. The first is the one you found to be incompatible, too bad since that's what I had my eye on.  The sleeve section looks a little short in that photo, was that the problem? (not inserting fully enough to achieve good contact)

If I solder up my own I'll probably order a short cable from CD International pre-terminated with female microdots on both ends, cut it in half and solder the two pig-tails directly to a modular plug or make a Y splice to a molded one.  The RG174 flexible cable they use is easy to work with and its center connector easy to solder.   I've made up a number of of extensions and fan-outs using them with no failures.

https://www.cdint.com/catalog/category/Cables/RG174+flexible

Edit- Hmmm, CD Int pricing has gone up significantly since I last ordered cables from them.  Still, I'll likely go that route given the quality, workability, and proven longevity.