In case anyone’s interested, I tried this route recently.
I went up Y Garn, then followed the ridge to Foel Goch using the usual path. From the end of that path, you can follow the edge of the cliffs back southeastwards (and Foel Goch is surprisingly impressive when viewed from there).
There are then actually two ways down. One is to carry on southeastwards directly down the grassy slope to the small tarn Llyn Cywion - this is quite a gentle slope, or (as I did) you can follow the ridge eastward around, which gives you great views of Y Garn and Ogwen. There is quite a clear path that follows the ridge around the upper part of the Mushroom Garden, then down into the cwm, which then meets up with the other path.
I think there a path to Idwal from here (but lost it), but it’s easy enough to fine your way down the grassy slopes to the old valley road.
Great route. The only downside is that it’s pretty boggy for most of the lower part.