When do the disk brakes engage in i-pedal mode?

I don't have an exact answer but I can contribute my experience. I use an OBD2 dongle to monitor data directly from the car.

Your point #1. Yes, I-Pedal is max regeneration, but up to a point. It seems that the limit is a deceleration around 0.2 to 0.25 G. Say you are at highway speed at 60MPH (97KPH) and release the accelerator for "full regen". I can see the deceleration sits at 0.2 G but the kW being generated is lower at higher speeds. As you slow down, the kW generated increases to maintain a deceleration of 0.2 G. (Point #2) Once you are down to much slower, city, speeds, yes, you generate less kW. A deceleration of 0.2 G at a slow speed would be almost like slamming on your brakes, so the G limit is lower at slower speeds. The I-Pedal max regeneration appears to be more of a bell curve in relation to deceleration, than linear with speed.

Point #3. When you gently apply the brakes, you will initially get some regen. This, I can see with the OBD2 dongle as kW generated. But you can also see in the dashboard that you are regenerating. As you step into the brake harder you will engage the friction brakes as well as increase the regen a little more. In older ICE vehicles you get little to no braking power when you gently put your foot on the brake pedal. In the Ioniq 5, it just adds in a bit of regen at first, then the friction brakes start to grab, then both as you step harder. The regen in the brake pedal smooths out what old friction-only brakes felt like. It gives it a more linear and predictable pedal feel. TLDR; Yes, brake pedal equals friction brakes. But also blends in regeneration, throughout.

I will also add that I have tried pedal braking with Level-0 (after the disc cleaning cycle) and I do see regeneration kW on the OBD2 dongle.