Awenau hear me
aid my song
As I write of the one
who is ever young:
Maponos, the son
of Matrona – Mabon
Who from Modron was taken
out of the light of the World
Into the darkness of unWorld
he was Maponos and not-Maponos
When the darkness held him
and he held the darkness about him.
Matrona – who is Being – searched
but he was in not-Being;
She was in every place in the World
but he was in not-World,
So she went there, and Being
was not, and light faded from World.
. .
.
Who was it held the key
to release them? Many stories
Tell of heroes in the spring of World:
Manawydan breaking the spell
That lies on the land
for Rhiannon and Pryderi to return,
Cei and Bedwyr riding the salmon
to find Mabon and bring Arthur
To release him from Caerloyw
out of the darkness into the light.
.
. .
O Awenau, he is the one
who holds the harp, the lute, the lyre,
He is the one who contains your song,
He is the muse of Fire!
.
awenydd.cymru 
I like the way you’ve brought the stories of Modron/Maponos and Rhiannon/Pryderi together here. I’ve also had a sense of Modron/Rhiannon being the one who originally searches for Her son rather than Arthur which is much later. I’d agree Manawydan is linked and that Llwyd is also connected as an Annuvian figure.
Yes I think the core myth is of the mother seeking her lost child which occurs in different cultures, though in some the child is
male while in others female. When the latter she becomes a wife of the Lord of the Underworld (as in the Homeric Hymn to Demeter, and he daughter Persephone), but when male he becomes a prisoner who has to be released or dies and has to be resurrected, as with the Egyptian Osiris.