No idea about the name or copyright of the two characters in the poster between Minoriko and Sakuya & Remilia's posters.
Oh and tried to find the socials of the illustrator of the 东方LiveParty poster, 白野威 but had no luck.

Alixiron said:

No idea about the name or copyright of the two characters in the poster between Minoriko and Sakuya & Remilia's posters

“红莲” and “小爱”,they are the original character of the ChongQing comicon “ComiTime”.
You can see the poster in(https://weibo.com/1969637871/Nfu9VwDFF)