QA hBP01-096〈Usada Pekora〉 with no fans attached was chosen for the ability of the bloom effect "Let's Make this the Best Concert Ever!" of hBP03-012〈Himemori Luna〉.
If I attach hBP01-123〈Wild Rabbit Alliance〉 to hBP01-096〈Usada Pekora〉 and use Arts, does the Arts get +20?

ANo, as that holomem did not have a fan attached when it was chosen for the ability of the bloom effect, it does not get +20.

QA hBP01-096〈Usada Pekora〉 with hBP01-123〈Wild Rabbit Alliance〉 attached was chosen for the ability of the bloom effect "Let's Make this the Best Concert Ever!" of hBP03-0012〈Himemori Luna〉.
A collab was performed, the ability of the collab effect "That is an "Adventure"" was used, and a die was rerolled for the ability of hBP01-123〈Wild Rabbit Alliance〉.
In this case, does the Arts of a hBP01-096〈Usada Pekora〉 without hBP01-123〈Wild Rabbit Alliance〉 attached get "+20"?

AYes, as the holomem chosen by the ability of the bloom effect gets "+20" if it has a fan attached, it does not need a fan attached when using the Arts.

QIf hBP01-123〈Wild Rabbit Alliance〉 is attached to hBP01-043〈Usada Pekora〉 who uses Arts 'Operation: Bunnify Humanity', how many dice can be rerolled using hBP01-123’s ability?

AIf the die was rolled 3 times, ignore those results, and roll the die 3 more times.