SCP Foundation

Event Description: Ten minutes before opening to the general public, three visit team staff and a security officer at ██████ Museum witnessed a Scutigera coleoptrata specimen, commonly known as a house centipede, emerge from a small drainage pipe in a storage closet. The specimen travelled approximately 1 m before entering a sink u-bend which had been opened for repairs. Specimen was estimated to be over 12 m in length, though of average width and height for its species. Specimen was visible for several minutes after the head portion had entered the u-bend, while the remainder of its body continued to exit the drainage pipe.

"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.