I'm not going to subscrube that Sandal is the Maker theory.
I will subscribe to that he is like Flemeth - a Dwarf who was left in the darkness of the Deep Roads ("That Aeducan boy who got lost a while back whose mother was a human or an elf" per Legacy DLC) and a spirit came to save him. I'm going to assume it's June - God of the Craft. Seen here with an Anvil - a common symbol in Dwarven culture.
The...slowness....might be of the interspecies mix...or could it be of something else?
Solas stated when referring to the death of his spirit friend that they could always come back if they leave a mark and memory strong enough. Though it may not be THEM. I'm likening this to a flower. The flower dies but what was once there might eventually grow back as something new.
So I propose this: I do subscribe that the Elven Pantheon and the Old Gods are one in the same. And June was one that has already risen as an Archdemon. A new spirit grew and took Sandal as a host. Because the spirit is so...young. We have Sandal with the mind of a child.
That or Sandal is an OGB...but considering the Carta members mention him as if it was recent that the boy went missing, I'm not banking on that one.