QIf hSD01-020〈Circle of hololive Listeners〉 is used and the die roll is 2 or less, does the Arts damage of hBP01-015〈Nanashi Mumei〉's Arts 'Oh,Hi' get increased?

AYes, Arts damage is increased.
Regardless of result, since the support card's ability is activated, it is considered used and Arts damage is increased.

QIf hBP01-015〈Nanashi Mumei〉 uses Arts 'Oh,Hi' when you have tools or mascots attached that are already active, will Arts damage be increased?

ANo, it is not increased.
Using support cards means using cards from your hand.