I don't know much about The Order 1886, but my advice is to take inspiration from it, not to try and rebuild it exactly.
I don't think you need a pop up juncture. 1850 is the Past Juncture and that is close enough to 1886, and removes all the limited time period questions that come with Pop Up Juncuters. Most portals in the Past juncture lead to China, but most isn't all. You could have a portal or two open to Europe, where some other faction(s) is getting a foot hold into the juncture besides the Ascended and the Guiding Hand.
In the Past Juncture the Ascended, AKA Transformed Animals, are dominant. Human/Beast monsters would scare the crap out of them in a variety of ways, as reversion is the thing they fear the most. Someone sending monsters like that would get their attention.
The high tech weapons speaks to me of Scrounge Tech, which makes me think there should be a connection here to the Future Juncture related factions. That means Jammers, Simians, or a rare survivor of the Architects of the Flesh.
Here is an idea: 1850 (Past Juncture) Europe houses a rare surviving Architects of the Flesh outpost, complete with mad science lab. Maybe the local portal to London was closed when the C-bomb went off, and only a few of the scientist were dusted because of it. Maybe the local secret lab made a habit of recruiting locals for lab assistants, so those are the survivors, while their local leaders turned to dust, along with all their demon-flesh Arcanowave tech. They also saw an example of Mutation from the C-bomb. The new leader of the local secret lab is a (locally born) mad scientist that has figured out how to duplicate some of the mutation effects of the C-bomb and is using it on trained animals to make monsters she can control. This has frightens the hell out of the local dominant Ascendant, because they think it's some kind of partial reversion of their own kind. Meanwhile, a portal to the netherworld has (re)opened in London, and some Simians have come through hoping to locate the Architects of the Flesh lab so they can claim the secret of causing mutations. They are the ones making Scrounge Tech weapons for the locals. Steam/clockwork weapons made from local materials and tech, but future levels of skill and knowledge.
So there you have it: A mad scientist using Architects of the Flesh technology and knowledge in a secret lab to generate mutant animals with which to attack the Ascendant, Simian agents providing time-inappropriate scrounge tech weapons and gear to local monster hunters as part of a plan to find the lost lab and it's mutation secrets, and the juncture dominant Ascendants that are frightened to death that the monsters might be their own kind in some kind of partial reversion (or worse, that the animal mutantion effect would effect them as if they were animals instead of people) and are therefore allowing the Simians to take point but are planning to swoop in and destroy everything with prejudice once the secret lab is found... even if that means killing a lot of innocents. Note that none of these factions care the least bit for the local people getting hurt in the cross fire.
Into the middle of this the players, ostensibly Dragons, walk. Look for a way to get some Melodramatic Hooks involved, preferably multiple hooks associated with different factions. As the faction of doomed heroes, they are likely to want to stop the mutant attacks, prevent the Simians from learning the secret of mutation, and keep the ascedant from blowing up part of London. Try to get the melodramatic hooks to align with that.
What we need here is a larger plot that needs to be stopped. What is the end goal of the mad scientist? As a surviving member of the Architects of the Flesh, she likely wants to see the Ascendant defeated, and maybe pave a way towards her own rise to power in this juncture. Maybe the intent of the mutation experiments is exactly what the ascendant fear: something that can force a reversion upon them. Right now it causes strange mutations in "real" humans, but turns animals into human/animal hybrids, but she has already discovered that it effects transformed animals as if they were animal instead of human. She is working on a new version that will be safe to real humans but turn transformed animals into their base animal forms. With such a weapon she could overthrown the Ascendant, and who cares if it has nasty side effects on the human population while she does so?
The finale might involve a fight on an airship (only a tad early) designed to spray mutagen onto London, which would be terrifying to the transformed animal population, but would also be a hazard to some percentage of the human population. (AKA: "Only 9% of the people will mutate, while another 23% percent of people will have a lethal allergic reaction to the formula. A small price to pay for Victory!") Ascendant have a nuke and are willing to blow up a chunk of London to eliminate the danger to their kind, and the Simians desperately want the formula that turns humans into mutants, even if it's just 1 in 10.
