wacchi-wachi?

Horo refers to herself by the first-person pronoun "wacchi" (another alternate pronunciation of "watashi").

This pic is nonsensical, but possibly a reference to Hatsune Miku's "Mikkumiku ni shite ageru" song.

It's likely to mean something.
Pixiv has even a tag for "わっちわちにしてやんよ".
And 585 hits for it in Google.
But no one seems to explain it.
It's related to Horo, probably a favorite expression of her.

Unlikely. It's just higher-order parody of a meme. XっXにしてやんよ is a meme, わっちわち is just a Horo version of it.