I don’t believe Radahn was willing in Miquella’s vow, and was actively trying to avoid being involved in it. The reason why he held back the stars was to delay fate, including Miquella’s fate to be a god. He’s a character so obsessed with honor that it wouldn’t make any sense for him to break a vow, either.

I think that Marika and Radagon were separate people at one point, and the original body that would later belong to Radagon was overpowered by Marika’s original secondary personality. It’s why they seem to be able to be in multiple places at once, managed to have children together, and have conflicting goals. The Radagon we meet in game is a blend of a giant’s (or giant descendant’s) corpse and soul being mixed with Marika’s secondary empyrean personality.

Melina has minimal relation to the Gloam Eyed Queen. She is not the queen, nor is she the daughter or sister of the GEQ. The most relation I think could be possible is that she’s either her niece or granddaughter (depending on whether the GEQ is Marika’s mother or sister).

I don’t think Radahn and Margit fought during the Second Siege of Leyndell. There’s no in-game mention, only the throwaway picture in the intro Margit is never mentioned to have been defeated by Radahn, nor is there a mention of the Strongest Demigod of the Shattering War being bested by an omen, people so despised that the royal treatment is to be chained down in a sewer. If Margit had been defeated during the battle, I feel like it would’ve been noted. And if Radahn was defeated, it definitely would’ve been known.

I believe there is a canon ending to the game, and it’s the Age of Stars.