You can move a gateway from a fleet to another fleet from:

1. The cart: add one or more gateway to the cart, click on the cart icon and then use the move action to move your gateways
2. The gateway overview page: go on the main page of the gateway you want to move and click on the floating button to edit the gateway. Select another fleet to move the gateway

You cannot. Gateways can only be in one fleet.

You cannot. End device can only be in one cluster.

You must have at least the role Admin to access to the user profile page you want to manage. But you can only deactivate or set an expiration date to a user who has a lowest role than yours. Of course, you cannot update the limitations of your own account.