Arcades's last ability doesn't actually change any creature's power. It changes only the amount of combat damage it assigns. All other rules and effects that check power or toughness use the real values. For example, Rabid Bite won't cause a creature with defender to deal damage equal to its toughness.

If Arcades leaves the battlefield after a creature with defender has attacked, that creature remains an attacking creature, although it will assign damage equal to its power.