International House of Patchouli

early joumon yukari, either way. Weird recently exhuman girl somehow became a more successful horror than any other youkai so they had to follow her example or get dead

They're not really being women in any concrete way. They're taking the form of human women, but to be a woman implies participating in the dance of gender that human society constructs. Which they don't really do; the "role" attached to the word "woman" is not something they participate in (at least in Touhou; some youkai do to an extent in legends, but even they don't fulfill the role entirely. See Kuzunoha and other fox-wives, or the tales of yuki-onna.)

I think you wouldn't really need the influence of Merry being a former human who still took that form in order for youkai to make the connection that "Ah, this form is associated with a nonthreatening and appealing role in human society. We should probably take it if we want to be more successful at preying on humans." Though there are other forms often taken by youkai in legends; salespeople, servants, monks, old women. Touhou characters looking like girls specifically is probably due to the types of legends that persisted into the Meiji era (a subversion of expectations has a tendency to be more interesting the more dramatic it is). And, in a Doylist sense, because all of Gensoukyou is an elaborate bit playing off of Lafcadio Hearn's illusionary Hourai. (Really obviously in early Windows, and still pretty obviously today.)

Basically, as long as people have made up legends about still being preyed on by things much bigger and scarier than the wolves and bears that no longer terrorize them, they've made up legends about those things looking like people. Merry looking like a people initially is convenient for her, but I don't think she was necessarily a trendsetter in that way.

Short version: one of the central tenets of Roman Catholicism is that God reveals himself to the world in three distinct persons – the Father, the Son, and the Holy Spirit – while remaining one God; this is known as the "Holy Trinity", and belief in it as "Trinitarianism".

The specific Trinitarian formula described above was formalised at the Council of Nicea in 325 AD, and all alternative formulations were denounced as heresy. One of those alternatives held that Christ the Son is a created being, subordinate to God the Father, rather than an eternal person of the Trinity; this formulation is customarily known as "Arianism", after its most well-known proponent, Arius of Alexandria.

Catholic folklore holds that Arius personally attended the Council of Nicea in order to present his argument; upon hearing it, Saint Nicholas of Myra – yes, that Saint Nicholas – became so incensed that he got up and punched Arius in the fucking face. Nicholas was subsequently imprisoned and stripped of his authority for disrupting the Council with violence, until a miraculous manifestation of Christ and Mary persuaded Emperor Constantine to pardon him.

Nonetheless she holds Marisa in contempt for that mess of a house, because at least she recognised that it was a problem and got help.