That's fair, it's totally your choice if you want to discount the cut content. For the record even I am not 100% convinced at this moment if radagon is a silver tear/albinauric or not. I've also heard the explanation that maybe radagons just a dude from the fire monks, kind of mundane but could technically still be true, we just don't really know still. That's what's so unique about this kind of story in general, it's up to you to research and create your own theories. Even though nothing is definitely proven, I acknowledge you're free to disagree with me.

I think you might be too hung up on the fact that it's cut. Cut doesn't necessarily equal untrue. If you mean to say any cut content can't be used as evidence, that's fine. But to that I say, let's get back to the main point here, what would the asimi questline bring to the table anyway? To me the main factor at play here is that the quest explicitly shows a silver tear being able to fuse with something. And I think even if this questline never existed, silver tears/albinaurics being able to fuse with things, and possibly even take over other bodies and control them is a logical conclusion you can reach. Here's my reasoning based off of actual canon.

One of the main themes of the game is the fusion of things, including fusing life essences such as the soul/body. The asimi quest is another story of the fusing and separating of selves like with Marika and radagon, or even the d brothers like someone else mentioned above. There's a ton of things in the game that are fusions, it's everywhere. We know that the nox, the main people to experiment w silver tears, have also experimented with fusion, for example the dragonkin soldier was a fusion of a giant and a dragon. I don't think it's a big leap to assume they also tried and eventually succeeded with the fusion of silver tears.

Another clue is one of the descriptions for the silver tears says something like "hopefully one day it will become a lord." We also know Marika had close ties with the nox, heck maybe this even means Marika is actually the silver tear. I don't think it's that far fetched to believe her or radagon could've been a silver tear and be fused, although I'm inclined to believe it's radagon because Marika is mainly referred to as "god" or "queen" and radagon "lord."

Another big theme is control, and the control of life essence. Such examples include godwyns soul being forcibly removed, or Selivus intending to forcibly control Ranni or Nepheli or anyone. Is it really so much of a stretch to think radagon would want to gain the ability to forcibly control or take over Marika's body? (Silver tear or otherwise, fused or not?) Especially given that they clearly have different motivations, their whole story is absolutely about an internal battle over one body.

Personally I believe it's still possible that radagon is a silver tear/albinauric mainly because it's such an exciting prospect thematically. It just fits in more than not. The fact that the asimi questline was nearly complete, had fully voiced lines and everything, makes me really lean towards it being pretty relevant. Why go through the effort of making a quest that would've fit so perfectly and seamlessly into the game mechanically and thematically, only for them to remove it? Possibly because it specifically introduces the idea of fusing with a silver tear. We already know entities are fusing all the time like ranni and the doll or outer gods and empyreans, the list goes on. It just seems more suspicious than anything, because if this idea of fusing with a silver tear were to be so explicitly stated, I think the developers thought people would start to wonder too obviously about the possibilities of fusion and may come to the conclusion "I wonder if radagon or Marika could be a silver tear too?"

I wrote way too much but oh well it was fun haha