1. To my knowledge, the return portal cannot be embedded but I do see the need for this ability!
2. Not currently, to show up in the returns portal, the shipment will need to be initiated in ShipStation.
3. split shipments and multiple package returns are not yet supported.
4. Yes, unless specified otherwise during your branded returns portal setup.
I would absolutely recommend reading over the full article on branded returns if you haven't had a chance to yet as well.
I hope this helps! 🙂