I'm pretty sure from tip to sleeve, the connections are:
Tip: Left audio
Ring: Right audio
So, whatever you plug into there as a mic needs to isolate the sleeve nearest the outside of the jack to connect with the microphone input pins.
I bought a little adapter thingy thinking that if I could hit the mic input I’d be golden. Turns out there’s some wack impedance autodetection in place and nothing I could do would make it detect that a line level signal was going in - it would always default to the built-in mic. I researched it loads and didn’t find many people talking about it or offering workarounds, and I eventually ponied up 20 quid for that cheap behringer USB-RCA interface. YMMV.
usb hub ftw. the ipad can handle a tremendous amount of things plugged into it.
at my peak of ipad integration I had usb hub into CCk with op1, midi keyboard, audio interface, and control voltage interface(cvpal) all plugged into the ipad at once and was able to get it all working no problem.
can still monitor out from headphone jack(most apps let you choose)
i used the ipad to midi control the op1 with keyboard and get tempo sync. the op1 now spits out a click clock which i can use to sync to my modular so no more ipad. and as per midi i use a raspberry pi. still looking to get an ipad pro and get my ipad integration going again. was such a pleasure to produce music on the ipad. totally serious in my studio laying tracks down and tweaking. laying on my couch tweaking sound lvls and fx. so great… wish it had more storage though…