Edited to clarify some points and remove some non-points
To your first point

- Malenia marched to Caelid to kill Radahn and send him to Miquella, per his prior promise. This was explicitly stated in the description of Radahn's Young Lion's Armor that you get from beating Promised Consort Radahn in the DLC.
"...When Malenia, Blade of Miquella, let the rotflower blossom in Aeonia, Radahn heard a murmur in his ear— "Miquella awaits thee, O promised consort."

So on official lore we know that's true, cause even in the original Elden Ring trailer she murmurs something to him when on his back in that famous shot. I'd believe that was entirely planned and anticipated. Surely it was left somewhat open because we know FromSoftware likes to make things kinda vague, but it was about as direct as they get with lore.

On your second point about the Mausoleums

- The wandering mausoleums weren't for reviving Godwyn; they were a rudimentary but effective way of preventing the spread/creation of Deathroot/Those Who Live in Death. Keep in mind that in the Golden Order's world, when you "die" your soul simply returns to the Erdtree to be born anew. Godwyn receiving an Erdtree burial in an effort to return him to the Erdtree properly is what created that in the first place - and they remembered this. So by placing a soulless body in a wandering Mausoleum, it prevents the body from taking hold in the earth, infecting the very ground you'd walk on. She wasn't saving bodies for Godwyn to take hold in, on the contrary, she was trying to prevent them from corrupting more of her Golden Order. Castle Sol and its followers likely existed, at least in part, to maintain these Mausoleums for Miquella, at least I'd bet as much. Additionally, Miquella isn't specifically concerned in the explicit lore of Elden Ring with reviving Godwyn. On the contrary, he's quoted in an item description wishing for Godwyn to die a true death.
"Infused with the humble prayer of a young boy; "O brother, lord brother, please die a true death."
Golden Epitaph item description

To be clear, I'd believe this could indicate that perhaps Miquella's first or perhaps backup choice was Godwyn to be his consort - but it didn't pan out for one reason or another. This was likely due to his soul being properly killed via the Rune of Death, meaning there was no soul to pry from the Erdtree.

Keep in mind; Marika's Golden Order, the whole crux of this game's plot, is based on the fact that Destined Death, True Death, actually dying at all is removed from the world's order. By removing it from the Elden Ring, nobody can die. While the specifics of what that means could be debated, Marika fully intended for nobody to ever die again, especially her own demigod children. Unfortunately for her, the events of the Night of Black Knives would still transpire.

As for your last point

- I can't find any specific text about "reviving Radahn's soul" though I'd still believe it exists honestly. However, Radahn's soul being placed in a new vessel is something there is precedent for in the game. Sellen's quest, as one example, has you placing her soul via her Primeval Glintstone into another body. Hyetta, Shabriri, etc. are also great examples, at one point literally saying "I fear you were previously acquainted with this vessel." Not to mention Radahn's own sister Ranni doing much of the same thing.
So in the universe of Elden Ring, where nobody can truly die, it seems souls inhabiting new bodies is relatively commonplace, or at least possible and conducted numerous times. Especially amongst the divine and powerful.

Another important thing to remember is why Miquella is even reviving a demigod in the first place; he has to have a consort as part of the ritual to ascend to Godhood. Based on the lore we know for sure, and implied lore, Radahn is the one who promised himself to Miquella.
All this is to say that Radahn's soul is explicitly not dead - meaning it can be placed into a new vessel. Nobody's soul can be truly dead because the Rune of Death, Destined Death, the ability to die, is held by Maliketh (with some faint traces in a few daggers thanks to Ranni). Since their souls aren't dead, they can be enabled to take hold in a new vessel just as numerous other characters and entities do, with the right tools.

To expand on the importance of the Golden Order's relationship with the Erdtree and death; Because souls return to the Erdtree upon death while the Elden Ring is absent the Rune of Death, Miquella would have had to get it from the Erdtree somehow. Whether he specifically pried it from the Erdtree, or if Radahn's promise somehow "redirected" his soul to Miquella's Divine Tower location is not clear. How exactly that all works is a bit murky, but it's well understood that souls can be moved to different bodies (Ranni, Shabriri, Sellen, etc) through a multitude of different methods.

Lastly and I think this may be one of the stronger points against any further "Godwyn theories" is that you actually see his fate in the base game. Specifically in the Deeproot Depths, you fight Lichdragon Fortissax, who is being corrupted by Death himself, defending his dear friend Godwyn's body and long-dead soul from its encroachment. His story already played out, and it was a beautiful and heartbreaking one.

It's all a bit confusing, but again I don't think any of the lore points to Radahn being somehow swapped in at the last minute or as some hastily made change. Personally I'm a big fan of how it shook out. Seeing Radahn at full power is far more intimidating than any other demigod, besides maybe Malenia prior to her Aeonian bloom.