Interesting theory regarding the giant figure
In Japan's creation myth, the mountains and their corresponding gods of Japan are made of the dismembered corpse of Kagutsuchi the fire god, after his father cut his head off because he burned his mother to death as he was born. Eight of them, specifically. and there's eight islands on Wano. and one of them has a huge severed skull.
and among these was the mountain/god of "mountains with iron in them". So if the islands of Wano are made of or otherwise contain the corpse parts of an ancient giant, then that could be the secret of seastone: ancient giants produced it naturally within their bodies.
Furthermore, some of the blood that sprayed as Kagutsuchi's head was cut off became a colossal catfish named Namazu that is said to live beneath the earth in underground seas and rivers, and cause earthquakes. That could be the giant shadow being spoken of. Namazu is said to have been pinned down with a giant rock by a thunder and warrior god, but on the days where the gods must all attend festivities in heaven, its power wanes and the Namazu can get partially free for a short while to cause a barrage of earthquakes.
Namazu's efforts might bring destruction and despair but he does have a positive side. The catfish represented the regular renewal of the world known as yo-naoshi which was welcomed by the poor as an opportunity to shake up the wealthy classes, redistribute their accumulated riches, and make a new start. This idea became especially popular following a series of earthquakes in the Edo Period (1600-1868 CE) which many times reduced the haves to the level of the have-nots and provided the poor with a momentary opportunity to improve their lot in the chaos that immediately followed such disasters