I used the word "Element" as a reference to the UML Element type, which is the superclass of everything you see in the UML metamodel. Meaning you can apply a stereotype on top of anything. (The MARTE profile has stereotypes that can be applied to all named elements)
If you have a stereotype extending Classifier, then all classifiers are valid to apply the stereotype to. (The classifier is the metaclass, the instances are any element in your model)
In EA it's more like adding a composite property to the instance that adds properties as defined by the stereotype. (ie. one of the common patterns for avoiding multiple inheritance)
Yes, primary stereotype as a metatype, and auxiliary stereotypes for for additional properties. (I'll call them facets)
And if you want EA to enforce that, set the strictness to profile for all of your metatypes. Then define a second profile in your technology for the facets. EA will then only allow you to define a single metatype, but you can freely apply facets.