An ambitious dream: Statting Touhou girls with 3.5

[AslanCross]

I'm afraid it does not, unless there has been some errata or something; it only increases spells per day and spells known.

It uses the exact same wording as the Incantatrix, Archmage, and all of the PrCs in the older books (such as Complete Arcane). If what you're saying is true, then the Incantatrix's brokenness is moot due to it having only a CL of 5 for the rest of its career.

I think the old wording has it clear in its intent that "as if you had gained a level in the arcane spellcasting class" increases spells known, spells per day and CL; the only thing you don't get is your old class's class features.

Anyway, re: Remilia.

Given her fluff that she's "the Devil," giving her the half-fiend template works well, so she can have wings. Half-Fiend also gives lots of spell-like abilities; I'd say her "Scarlet Devil" spellcard could be modeled by blasphemy.
-Half fiend also gives: claws and an actual bite attack, neither of which vampires get.

I'd say she has Warlock levels, and that her projectiles are variants of Eldritch Blast. Having levels in Hellfire Warlock powers up her blasting even more.

-Eldritch Glaive and Eldritch Spear could model Gungnir for both melee and throwing.
-flee the scene invocation for teleportation. She can do it at-will and leaves behind a major image.
-hungry darkness: shadows filled with a swarm of bats. Can't get any more thematic than that.
-dark discorporation: turn into a swarm! Of bats!
-dark foresight: very good buff that works on self and others.