"As and when" here messed with my head, even if made sense after checking and rationalizing definitions.
Also, Mobile Task Force is singular:
As and when it re-manifests, Mobile Task Force Upsilon-Peorð ("Slings and Arrows") are is to track and sedate the entity
You abbreviate one unit and don't abbreviate the other. I suggest you keep it consistent:
at least 1.5 km kilometres in length and around 5 metres in diameter.
'Close' analysis doesn't sound clinical. 'Detailed,' maybe? 'Proximate?' Something more precise:
Close Detailed analysis of SCP-3MIL's biology has not been possible
This should be made its own sentence, and described passively, not with the imperative mood:
non-anomalous millipedes. It should be noted that, despite this, it is a genetic match with Harpaphe haydeniana and externally identical, excepting the additional body segments.
'Location' should be plural:
otherwise move quickly between locations.
This should be a footnote – as it is, it breaks the flow of an important sentence. This also calls for the removal of "that is to say," which I'd avoid in clinical writing:
That is to say A parallel universe that differs from baseline reality in one minor aspect, usually a different outcome to a specific event.
Establishing a parallelism here:
These wormholes are two-dimensional, largely imperceptible, and remain for some time temporarily stable after their following creation
Colon shouldn't be used here. You could replace it with a semicolon; but this sentence is so long, I recommend cutting it off with a period:
which organisation should take control of the anomaly. As per the Boring Agreement, Wilson's Wildlife Solutions had area priority
I think the record of all major events should be catalogued as an addendum.
Here, you can remove 'that' because it would be implied. I suggest you remove it because you've already used 'that:'
members of SCP-3MIL's containment team believed that US President William Henry Harrison died of an infected wound
"Containment lost" is a weird phrase. You should say "SCP-3MIL remains uncontained" or "breach lasts" or some other statement that SCP-3MIL isn't contained at this point:
Containment lost Breach lasts for approximately six weeks
I think this should be "places its date at." I can't cite any specific rule in grammar for this because I can't find one, so don't quote me on that. Here's how it would look:
Preliminary analysis places its date of creation at no more than five years previously.
'And' muddles causality here. Say "after which:"
Numerous crossbow bolts are embedded in the entity's back, and after which it appears severely weakened from extensive blood loss.
This is a roundabout way of saying "she died." The Foundation does not use euphemisms. Say "she died:"
a woman by that name had approached him for a position some years previously, but had been involved she died in a fatal car accident shortly afterwards.
The two things the Foundation wants to locate are PoI-7643 and the WWS team. Restructuring this sentence would make this point clear, and the writing more concise. Saying the location of the origin is redundant – 'origin' is defined as a place, so this is like saying "the location of the place." That the origin might be another universe is inherent. Here's how I'd rewrite it:
The location and origin of PoI-7643 ("Kimberly Madison Alternate") and the WWS research team responsible for initial categorisation and their universe(s) of origin are currently unknown.