Yes, the blue wire in the green harness.
Yes, the idea if I understand the function is to use the aux input input to trim flow.
I am using an Innovative LC-2 for wideband o2. This device has two configurable outputs. The first output (a yellow wire) provides wideband o2 signal to my Diablosport Trinity for data logging via a 2.5mm jack. The second output is unused and configured for lambda by default but can be reprogrammed with a PC to be a 0-5v wideband output instead like the first output. I was thinking to just link this second output of the LC-2 (brown wire for the lC-2 if I recall) directly to the HFS-4 blue wire. This would of course be a soldered connection with heat shrink tubing and all that good stuff.
I can tie into the OEM IAT wires as an option if I want to use that signal instead.
edit: The first iteration of my install with be water only and no tune so no failsafe. I may advance the ignition timing on my own using the Diablsport Trinity, though. I will eventually work in a failsafe using a 3 way solenoid to cut boost and then get a custom tune for 50/50.