With how quirky they're portraying Frill Shiranui, it kinda feels like she's an option too. Not just an actress on a holy pedestal but this quirky character

It feels like her joining would both remove the tension with Kana about Aqua, but introduce bigger tension between Kana and her because she's a more successful actress. Neither of which seem like a good direction for the story.

Don't think so. 3 is a good number already, I guess and they probably don't want the story to revolve around new B Komachi too much?

Nah, I feel like she'll stay an actor to remain in Aqua's story, rather than move to Ruby's.