ConProcs:
LBL:
Main storage of SCP-XXXX-1 instances is at Site-11
This is more of a personal preference, but I'd phrase this as something like "Additionally, instances of SCP-XXXX-1 are to be primarily stored at Site-11".
MX-Qualification are encouraged (but not required)
Another personal preference, but you could put the parenthesized portion as a footnote.
Content:
Narratively, the ConProcs you've listed are very solid. They portray the Foundation in a very competent and professional manner, with some hints at shady, veiled activities towards the end. One thing I would clarify here though, is what MX-Qualifications are. While it's certainly clear what the qualifications grant access towards, there isn't really an explanation as to what they are. I'd personally add a brief snippet as a footnote, mentioning what they are. Other than that though, the rest of this section looks good.
Description:
LBL:
SCP-XXXX has led to many important results. It is not uncommon for unproven conjectures in traditional mathematics to be proven using SCP-XXXX.
Personal preference, but I'd slightly rephrase this as "The discovery, and subsequent use of SCP-XXXX has led to many important results. For example, it is not uncommon for unproven conjectures in traditional mathematics to be proven using SCP-XXXX.
The Riemann hypothesis
the Riemann hypothesis
becoming busy with something else,
I would drop this portion, personally. It feels a bit too informal.
Content:
This section is also really solid as well. I find it interesting how the presentation and use of the anomaly is practically in plain sight through the use of both verbal and physical language, yet only accessible to those with innate understanding of it. I also like how you've incorporated it into both real world conjectures, and alternative history.
Addenda:
LBL:
I must have so so much catching up to do.
I'd either do "so, so much-" or drop one of the so's, since it's a bit confusing to read.
Content:
The addenda are a bit of a mixed bag for me. They definitely start off strong, giving good reason as to the significance of the GPS trackers from earlier. The disappearances have an unsettling, almost cosmic horror vibe to them, as if the researchers peered too far into the unknown and got lost in it. I also like how it's essentially resistant to amnestic treatment as well. It definitely plays off that earlier cosmic horror vibe, and makes it feel like once you've this far, there's no unseeing that hidden pattern.
Where it kind of derails for me is in the final two addenda. It undoes a bit of that unsettling, subtle eeriness through Dr. Thomas' essentially unfazed return. While the increase in disappearance percentage at the end does imply that she probably interacted with or learned something that is causing this similar pattern in others, it comes off as a bit jarring.
Personally, I would devote most of the addenda to exploring the character of Max Willow, especially post-amnesticization. You can still keep the section of Drs. Moore and Thomas disappearing (possibly as part of the reason why Willow wants to leave), but then focus the follow-up addenda on Willow's blog posts and interactions with the Foundation. You can have Willow try to pick at where these unknown thoughts and patterns are coming from, and what he thinks he's starting to see through it, despite Foundation intervention. You could eventually lead all this up to some sort of discovery Willow makes before disappearing himself. This could effectively achieve the same outcome as your current Thomas ending, in that there's some piece of knowledge out there that is triggering all these disappearances, but give it a little more insight as to what it could be that they're seeing.
Final Thoughts:
Overall though, I think this is a very fascinating concept. I think you've definitely captured a sense of some eerie, unknown anomaly that is present everywhere, yet seen only by a few. In regards to your concern over the word choice and tone, aside from the few portions I highlighted above, they were both relatively solid. Going forward, I would focus more on both expanding Willow's character, and exploring/hinting at what this phenomenon is that the MX-Qualified researchers keep encountering prior to disappearance.
