QWhen you have holomem A in your center position, if 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 still keep Arts+20?

AYes, the holomem still keeps Arts +20. After bloom that holomem will still retain the effects.

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.