QIf Kaguya is on my field, and I play this amulet, is it possible to banish an enemy Electromagical Rhino with 7 defense with this amulet's Fanfare ability?

AYes, it is, but only if you resolve the abilities in the correct order. Kaguya's non-Fanfare ability and this amulet's Fanfare ability are triggered simultaneously, so you may resolve them in any order. To banish an enemy with 7 defense, you must first resolve Kaguya's ability to deal 3 damage to that enemy. Then, since that enemy now has 4 defense, you can select it and banish it with this amulet's ability.