I got lost myself doing this quest a lot. I ran back to Y'shtola about 3 times before I found the other hallway. >.< (I tried to use the map as reference the whole time, which threw me off)

Thankfully, I was on my WHM so I just Bene'd Ser Aymeric after I found them all. I think he was at 16/100 when I got to him. XD