QMy hBP04-047〈Yukihana Lamy〉with hBP04-106〈Yukimin〉attached collabs, and my holo Power becomes 3.
In this case, can I use the SP Oshi skill "Vroom, Bari Bari Bari Bari" of hBP04-004〈Yukihana Lamy〉and get +100 special damage before resolving the collab effect "fleur"?

ANo, you cannot.

QI used the SP Oshi skill "Vroom, Bari Bari Bari Bari" of hBP04-004〈Yukihana Lamy〉 on a hBP04-047〈Yukihana Lamy〉with hBP04-106〈Yukimin〉attached, and the collab effect "fleur" after that to deal 120 damage to my opponent's holomem.
If I bloomed to hBP04-048〈Yukihana Lamy〉and used Arts "May Today Be a Blessed Day" during the performance phase, does the ability from Arts get +100?

AYes, it does.

QI used hBP04-004〈Yukihana Lamy〉's SP Oshi skill "Vroom, Bari Bari Bari Bari" and chose hBP04-047〈Yukihana Lamy〉.
If I bloom it to hBP04-048〈Yukihana Lamy〉after that, what happens to the SP Oshi skill "Vroom, Bari Bari Bari Bari"?

AThe ability will be carried over even after blooming.

QI used hBP04-004〈Yukihana Lamy〉's SP Oshi skill "Vroom, Bari Bari Bari Bari" and chose hBP04-043〈Yukihana Lamy〉with hBP04-106〈Yukimin〉attached.
Then, I bloomed to hBP04-047〈Yukihana Lamy〉, used collab effect "fleur", downed my opponent's holomem and drew 2 cards.
In this case, can I use hBP04-047〈Yukihana Lamy〉's Arts "Bouquet of Shimmering Snow" during the performance step, down my opponent's center holomem and draw 2 more cards?

AYes, you can.