QIf your Life is 2 or more and your hBP01-014〈Amane Kanata〉 with hBP01-116〈Upao〉 attached is downed by opponent’s center holomem's Arts, can you deal damage to opponent’s center holomem due to hBP01-116〈Upao〉's ability?

AYes, damage is dealt.
hBP01-014 〈Amane Kanata） receiving damage triggers hBP01-116 〈Upao）'s 'when the holomem this mascot is attached to takes damage' ability, which goes into standby.

Then, rule-based down resolution archives hBP01-014 and hBP01-116, and resolves that standby effect of 'when the holomem this mascot is attached to takes damage' at the check timing.

QIf your buzz holomem suffered damage exceeding remaining HP from opponent's hBP01-014〈Amane Kanata〉's Arts 'Wings Darker than Black', how much life damage do you take?

AAs hBP01-014 〈Amane Kanata）'s Arts 'Wings Darker than Black' fulfills the condition, the opponent's life gets -1, and the buzz holomem's extra ability 'If this holomem is downed, you get life-2' activates, resulting in a total of 3 life damage.

QWhen using hBP01-014〈Amane Kanata〉's collab effect 'Fallen Angel' to down opponent's center holomem, can you deal damage to opponent's life?

AYes, it is possible.
Even if you down an opponent's holomem with special damage (and there is no text indicating Life increase or decrease like 'your opponent's life does not reduce even if downed'), you can reduce the opponent's life the same way as when down is caused by Arts damage.