Does it make sense to introduce more API dependencies when we are in the process of moving them out? Let's just create a generic observable for all types of messages?

What do you mean? Is there a PR where we are moving out the API dependencies? Do you mean changing this to the following?

/** @private {!Observable} */
this.scrollDocObservable_ = new Observable();

There'd be a map of observables: !Object<string, !Observable<!JSONType>>. The index is the type of message. The API will be viewer.onMessage(eventType, handler). The records in the map will be created from calls to this API. Thus we simply remove a switch if (eventType == 'x') {x.fire(payload);} to simply if (observables[eventType]) {observables[eventType].fire(payload)}.