Radahn had nothing to do with radagon leaving her, this is a ridiculous take.

Martin is big on writing about characters that are threatening in battle being defeated through their love of someone.

Radagon meeting her and her just happening to fall so madly in love that the war ended in favor of Marika is highly suspect, especially when you consider things like how malenia spawns offspring through buds, and the fact the miquelle is well known for supernaturally compelling loyalty.

It isn’t flat out spelled out but it’s highly likely radagon was just waiting for marika to call him back the entire time, even if he wasn’t created by her in the first place.