"and finally exit the ruins."

It's worth noting that, if I'm not mistaken, this will allow you to explore the last question map marker in the entire game (not counting the DLC, of course).

"Turn south-west and blast some stalagmites and stalactites with Aard to expose a chest, then continue west up another flight of stairs."

...then continue east up...

"When you reach a statue turn south-west to find a chest worth looting"

there is another chest also to the south-west, but in the water before going up to the statue.