Some basics:

Radagon is male aspect of Marika. Two souls, one body. They have different goals and are not 100% aligned. Also Radagon is not something that was created by rune.

Elden Ring doesn't need to be shattered in order to change it. Elden Lord and likely Marika can alter the Ring. Marika is a vessel of Elden Ring.

Elden Ring is definition of Order written by magical runes. It requires a vessel, which needs to be an Empyrean. Empyrean vessel of Elden Ring is a god. Gods in Lands Between derive their power from Outer Gods, who are powerful entities who can't directly influence the world. Hence the need for vessel. There is evidence there were different Elden Rings and different gods before Marika(lore of Crumbling Farum Azula and Placidusax).

Grace isn't granted by Marika but by Greater Will. Marika can divest people of Grace(likely by banishing someone outside of Golden Order) but there is no evidence she can bestow it on her own. She claims she can, but the power of Grace comes from Greater Will, to which she is subordinate.

Marika IS Radagon and Radagon IS Marika. They only have one body. That's THE problem. Unlike with normal Elden Lord - say Godfrey - Marika can't negotiate with Radagon. That is not a problem while their goals align. Once Shattering occurs we know for a fact that is no longer the case. Marika Shatters the Ring and Radagon takes over and tries to fix it. When in form of Marika - only Marika is in control, in form of Radagon - only Radagon. Once the Ring is shattered, Marika is punished by Greater Will and crucified inside the tree - she is now merely a vessel, not an active participant. There is Radagon's seal on the Erdtree - this indicates Radagon is still aligned with Greater Will and current version of Golden Order.

There are more bits, but basically if you have a theory it has to encompass all the lopre and all the evidence inside the game. If there is something in game that contradicts it, unless you have a good explanation, then the theory is wrong.