QIf you have 〈IRyS〉 in both center and collab holomem positions and both bloom to hBP01-030〈IRyS〉, does bloom effect 'Let's Fill the Stage with Hope!' activate twice totaling +60 Arts to center and collab holomem?

AYes, the effects stack and total Arts damage is +60.

QDoes 'During this turn, your [center holomem and collab holomem] with #Promise get Arts+30.' from hBP01-030〈IRyS〉's bloom effect 'Let's Fill the Stage with Hope!' also apply to special damage from Arts text?

ANo, it is not applied.