QDoes removing a Magical Item from play after playing it as a spell card count as banishing it from my EX area?

ANo, it doesn't. When a token leaves the field, it is first placed into the cemetery. Then it is removed from the game. Therefore, since the Magical Item is removed from the cemetery after being played as a spell card, it isn't banished from your EX area.

QIf there are no selectable followers on my field, can I still activate this amulet's ability and bury it?

ANo, you can't. If an ability requires you to select a target, and there are no valid targets on the field, that ability cannot be activated at all.