I have to say, you have a very great way of using imagery. If I can focus on the Addendums here, I want to say that the dialogue between the D-Class and Security is very casual, especially with the expletives. It's understandable that the security would be very annoyed at D-Class, but intense sarcasm to their face might not be something that'll fly with a researcher on the scene who can get them a reprimand for it. Since the security are, well, security and not MTF on a high stakes mission, I can get behind a little slacking and minor tomfoolery, but having security guards blatantly ignore the screen for signs of trouble in favor of a random smoke break might be hard to get behind for some.
I'd say the dialogue for Dr. Calvin is fair enough, though near the end when he's conducting the final experiment is secrecy from his supervisor, I think it might be worth adding something about how Calvin might've been punished for an unapproved experiment + the fruitless loss of a D-Class, along with how the experiment was documented in the end anyways for the new light it shed on the SCP.
And one last thing that's affecting the addendum on the whole; you write as if this is a tale, and not a report on an official document. The clinical tone is lacking, especially at the end of the final addendum. Specifically this bit;
[The desperate banging upon SCP-XXXX's door ceases, and is in turn replaced by absolute silence.]
[Around 20 seconds later, blood begins to flow from the bottom crack of the door onto the floor of SCP-XXXX's containment cell.]
[The sound of silence is then replaced by the hushed dripping of blood, a dark crimson, slowly pooling in front of SCP-XXXX.]
[Suddenly, the flow stops, and the sound of silence returns to SCP-XXXX's containment cell.]
[There is 5 seconds of painful, empty silence.]
[Then, the terrifying 'click' is heard.]
[SCP-XXXX's door can be heard being unlocked.]
[Both security officers appear frozen in shock, barely able to hold their weapons.]
[A minute passes, and Officer Thompson takes a step forward. Another step proceeds that step.]
[Quickly enough, Officer Thompson reaches SCP-XXXX, and places his hand on the doorknob.]
[Officer Thompson slowly pulls the knob down, as the door slowly screeches open.]
[The room appears empty, though with a visible bloodstreak trailing to the closed hatch.]
[Officer Taylor is seen reluctantly approaching the hatch, before pulling it up to ensure it was locked.]
[Just as the hatch swung wide open to Officer Thompson's dismay, the door to SCP-XXXX slammed shut.]
That's really good imagery, like I mentioned earlier, but it's less like something you'd read in a official document and more like something you find in a horror novel.
Overall I really like your idea and hope it does well if you ever upload it.
