To me every Observable seems like a piece of a state (aka object) which sends messages (events) to other Observables (objects).
If so, question arise:
* When to use "raw" objects which respond to events through their methods (1 method ~= 1 event), and when to use Observables?
* Is there anything an Observable can't do but a "raw" object can? Is there anything a "raw" object can't do, but Observable can? Simplicity, expressiveness matters!
* Shall I replace every object with an Observable? Are Observables "just superior" in every way? If not: what are the exceptions?
An observable can be created from an event and have several transformations applied to that event (see https://www.learnrxjs.io/). These transformations functions are powerful because they can be composed to create the observable and can also be shared and reused.
They are not "just superior in every way", they are just suited to listening and emitting events to/from various components in your system. A simple object is perfect if no events are needed.
I would describe Observable as representing an uninitiated computation. More analogous to function reference than an Object with state. If you delve deep into an observable implementation you'll likely find that the only concrete property is the function to invoke on subscription.
As an exercise replace "Observable" with "function" in each of your questions and see if they still make sense.