First of all, you clearly put a lot of time and effort into this. I feel like YOU, the author, care about these characters and what happens to them. That's a good feeling to have while reading a story, so I thank you for that experience.
I also loved the beginning of the article and the way it frames the lake as some kind of miracle water, healing the wounds of forest creatures. That automatically introduces some tension, because I suspect things aren't what they seem.
I think my favorite moment in the entire story is when the doctor asks D-23682 to relate his encounter with SCP-9523 in his own words, and is surprised that the first thing he mentions is how anxious he felt around all these armored guys with guns (including a guard who hates him) walking him through a natural environment he's had little exposure to and has lately been cut off from. That's just…so good. It underlines that there is a *total power imbalance between the subject and the doctor*, no matter how kindly the doctor behaves, and reminds us that the doctor isn't truly empathizing with the other man at all. Exchanges like this are much more powerful than characters outright stating how they feel/what their motives are.
This power imbalance and the idea of this D-Class as a vessel for the doctor to work out his own trauma—with no real regard for who the other man is and what he wants or respect for his agency—is the strongest part of the story.
I also loved the haunting imagery of the hooded figure. I don't like when the supernatural is world-built into something quantifiable and unremarkable, so I thought their presence and mysterious motives were great.
I do feel like certain aspects of the story are belabored to the detriment of the whole. I'm going to be specific about these points, since they're why I'm stuck on a novote.
I didn't need a page-long psychological profile of D-23682 AND his first interview with Dr. Trautmann to understand who these people are and their connection. Nate's issues and the doctor's solicitousness are sufficiently communicated through their interactions. Even there, some things are belabored—like Trautman saying, "You know I have a vested interest in your wellbeing" when he's already clearly communicated that interest.
This re-statement of themes is at its worst near the end of the story, when the doctor outright states,
A selfish, grieving part of myself, the one I have been suppressing for the past year, wanted to keep you by my side. And, I gave into it. I caused you unimaginable harm to you.
This is way too overt! Same with Dimov saying,
You are unraveling, August. When you lost your son, you refused to take time off to heal. And now, the project has faced the consequences of that action.
This is pretty classic telling vs. showing. You've GOT to tell some things! But in this case, we're being told what you've already shown again and again, so the truth begins to lose emotional impact.
The emails about whether Nate will be selected for testing (and most of the back and forth emails) go on for wayyyy too long, specifically because most of them *aren't introducing new information*, they are just repeating information we already know. There's potential danger from the SCP. Nate is troubled. The doctor is weird about Nate because of dead son. Nate will encounter the SCP and bad things will happen. This whole exchange could have been one punchy, "D-23682 confirmed for testing at Dr. Trautmann's request". Also, I get suspending disbelief for the sake of the story, but folks aren't writing prose in Outlook!
I also didn't buy that staff would view Nate as uniquely "troubled" or "volatile". You present a world where D-class can rot while living and the Foundation hems and haws about morphine costs. How many D-class in this context are NOT volatile, depressed, marginalized and dehumanized? I know this is meant to play into Nate's backstory, but that connection also doesn't land—maybe because the main connection that's explored is between Nate and the doctor, NOT Nate and Tomas.
The backstory with the break-in and extra elements like the 16th century journal and Nate's final note just end up feeling perfunctory. I'm not sure what they were meant to add to the story.
I think a lot of readers are going to have no issue with these things. I'm just commenting on them because I appreciate the effort that went into completing this story. I hope you feel pride in your work!