Hard to say. I did a bit or searching, unfortunately there's not a lot of info about them. In particular what kind of sensors are in there and where they are positioned.

All MIDI Expression devices have had buffered inputd for the past several years already.

The jacks can definitely share the same ground. I'm not so sure about about the power connection, but I suspect you can. You'll definitely want to have the polarity of the inputs set though (not auto)

Crosstalk cancelation is applied to the MIDI coming the devices connected to the USB host port as well as devices connected to the MIDI port.

My studio is out of commission for a week or so, bit I'll look investigate this when I'm able.

Perceval was trying to help you, and my answer help is going to be similar to his, so I apologize in advance for being obtuse. eDRUMin devices don't produce 'audio', they produce MIDI. You need some sort of plugin to produce the audio (Superior Drummer, Steven Slate drums, Addictive drums etc.) I do...

Yes.

I haven't really read your post all the way through but the first thing that comes to mind is hihat splash, maybe your hihat splash nite is the same as your cowbell.

Also you can use the midi monitor to track down the source.

Yup, I can confirm this is a bug on macOS / Linux. Fixed for the next build. Thanks.

Update: I posted build 4 with the fix.

If you look at the cable in the picture, you'll see that it has 3 TRS jacks, so the single input MIDI Expression is not going to work. You'll need to use an iO or Quattro.

eDRUMin devices are incapable of damaging your cymbals. I don't know of any combination of settings that could cause a problem. You have the pads, and you have the devices. You should be able to figure out what's going on.