To give a more clear sense of where I am currently - I was able to confirm line level passthrough is working just fine using the tip from @Ilias here:
https://forum2.elk.audio/t/cant-seem-to-get-audio-input-going-with-fx-demo/553/14?u=kcoul
But of course the main difference is that I want to use mic level with controllable gain, and in app context rather than a plugin.