While resurrecting Jack, Castiel had to travel into Heaven itself to find Jack's soul in order to send him back into his body.
The grace can then be consumed by any angel or placed inside a vial.
The best soil options should be well-draining, peat moss based, and relatively rich in organic material.
Skip was found abandoned when he was six years old, and now resides at Pacific Sanitarium, which is seen as a last resort for children like Skip.
How does destroying angel kill? If you really wanted to you could steelman the point that Beerus is stronger but you'd have to rely on pretty weird arguments to do so.
Raphael was capable of making Crowley immune to Castiel's.
The original post also suggests that a being carrying Nephilim genes might also be inflicted or blessed with 'Synesthesia'.