No. That would be a breach of the ThemeForest license. Technically speaking, nothing prevents you from doing that, but you would be breaching the license agreement and could lead to other issues. So it’s up to you to decide wether you want that or not.
No there aren’t. ThemeForest has two license types – Regular and Extended, and both restrict the use of a single license per domain. The Extended one only refers to redistribution of the theme. More on this topic here https://themeforest.net/licenses/terms/regular .
You can’t have the same purchase code active on multiple sites simultaneously. So for instance if you have a localhost or staging site, you will need to deregister the purchase code first and then activate it elsewhere.
Besides what’s needed to be pulled from the cloud, everything else should work normally. So basically updates, getting the demo content, downloading Rey’s bundled plugins or getting template items from the library, will be avilable only when the Rey instance is registered.