Thank you for your hard work.
As for "yappi as greeting", a quick search gave me two suggestions:
Holofact : Tokoyami Towa's greeting "Konyappi" can be divided into 2 words "Konbanwa" a Japanese greeting used in the evening and "Yatta" or "Yay" in English, but Towa got into a habit of saying "Yappi" which leads to her famous greeting.
and from
"What does she say in Japanese that gets translated to Hiya Papaya": Yappi~! or Yoropippi~! and whatever (basically adding っぴ or ぴっぴ to various words). It's funny cause I always see the first few replies to 5channel engage threads being stuff like 乙ぴっぴ or スレ立て乙ぴっぴ
(also
https://virtualyoutuber.fandom.com/wiki/Tokoyami_Towa says about yet another person that She greets her viewers with "Ohayappi" (おはやっぴー) or "Konyappi" (こんやっぴー) and she says goodbye with "Otsuyappi" (おつやっぴー). )
So I guess it's kind of a facade for what they want to convey, and in this case it's something like 'sup.