When you hold shift key,:

Is it possible to cast an ability then cast another like a structured queue?

Is it possible to assign a generic unit an order to go somewhere, cast something, then order that unit to come back? If so, will the unit wait until the energy is restored, and once it's enough, the unit will cast the above ability, then follow the next order?

Can the queue intertwine with other shift queues?

You can order a worker to build something, then move to a different location, then build something else, and then start to gather resources again. That's just an example of the queues you could set up with the shift key.

If you order a worker to build 5 gateways, you will have to pay for these gateways immediately.
The won't build one gateway, then wait for you to collect enough minerals, and then build another one.

I hope this answers your questions.