QIf you increase Arts by +20 for holomem A with hSD01-004〈Tokino Sora〉's collab effect 'Let's Dance!' and then bloom holomem A to holomem B, does holomem B keep the +20 Arts increase?

AYes, the Arts still has +20.
The center holomem remains on the center position, so the effect continues even if the holomem changes.

QDoes the collab effect 'Let's Dance!' of hSD01-004〈Tokino Sora〉, which states 'During this turn, your center holomem gets Arts +20.' also apply to special damage described in the Arts text?

ANo, it does not apply.