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But how do they validate the request came from the sender?

They probably only allow it for business customers. In which case there's a shipper account associated with the tracking number and they have online access to view/manage all outgoing parcels/deliveries.

This used to be called Quantum View; a quick google suggests it still is.

You create an account if you are sending online. The shipment is linked to that account. Then when you ship in-person you can add those shipments to your online account via some id on the shipping receipt last time I checked.

It depends on the terms of the shipment; if you're paying the shipping company, you can choose who can redirect it.

I routinely rerouted my packages in San Diego since UPS wouldn't attempt delivery. You can do it with most packages unless the seller specifically has specifically forbidden it, (thank you Ingram Micro for sending me on a 3 day chase for my package)