Drumster I'm mixing on a Mackie ProFX8v2 USB and have my iPad Air 2 connected to Ch.1 in mono by just using either the left or right mono jack plug from the stereo cable. But my tracks are mixed in mono, so this is safe. Obviously (or not), make sure you're not connecting up a stereo cable from the iPad to a mono channel. First thing I noticed when switching to ST3 was a 7dB volume drop due to the default headroom in the app for the volume sliders, which I read you know about already. On the mixer, channel gain is on Unity and the fader is on about +5 at the moment, but I believe I could go back to around 0 using the +6 boost on ST3's master EQ. I really haven't tried USB with the only control being one knob because there's no way to route that to my monitors as far as I know. The manual just says USB routes to Main. I no longer have a USB Camera Connection Kit to test (I bought one for my iPad 1, but then Apple discontinued and disabled it for a while so I threw it out), but I could try USB on my iPad Pro which has a USB-C port. That might be comparing apples to pineapples.
Anyway, not sure if any of that helps except for the comment that the mixer (v2) works fine for me with ST3 and iPad via 3.5mm output.