Plargg, Dean of Chaos’s second ability allows you to cast the spell during that ability’s resolution, ignoring its normal timing restrictions.

Despite the term “other tapped creatures,” Augusta’s first ability doesn’t care whether Augusta is tapped or untapped. The same is true for the second ability. Both abilities apply as long as Augusta is on the battlefield.

Untapping an attacking creature doesn’t remove it from combat. It’s still attacking normally.

Augusta’s last ability triggers whenever you attack with any creatures, even if you didn’t attack with Augusta.

Augusta’s last ability untaps all creatures you control, not just the ones that attacked this turn. Then you choose which creatures to tap, if any.