5) Use the component input (YPbPr) on the OSSC… as before… and feed the audio into the OSSC on the “A1 OUT/A2 IN” connector via RCA R/L to 3.5 stereo adapter.
That’s what the “A1 OUT/A2 IN” connector is there for. You can use it as analogue audio output connector to pass through audio from “AV1” (SCART)… OR you can use it as audio input connector for “AV2” (Component/YPbPr). As written in the Wiki:
Analog audio from SCART input is bypassed to a 3.5mm stereo output jack next to video output connector. On v1.6 boards the jack alternatively functions as AV2 audio input, selectable via a miniature switch.
… you have to use the miniature switch (on the right side of the “A1 OUT/A2 IN” 3.5 mm jack) to switch between A1-out/pass-trough and A2-in mode.