Sure - if you want to trigger a rule as if the scene was triggered over Z-Wave, then you should just set the item to the value of the scene - so what you have proposed should work fine if I understand you correctly.
No - this is 100% down to the device you are using. Often a scene will be linked to a button on a remote controller, but this is defined by the controller - not zwave.
I mean the zwave scenes which I can configure via Habmin. See screenshot
There must be something like a zwave “general Alarm, water flood alarm, …” scene which can be published to the zwave network. An actors can react on them. Are there general scene numbers or whatever available to publish this to the zwave network.
Same with siren, they will react on the specific scene which is published. Or am I wrong?
No - there’s not. As I said, a device will define this. For example, in a remote control, if you push button 1, it might send scene 1, push button 2 and it might send scene 2 etc…
You have listed notifications - this is not the same as scenes. For notifications, there are standards defined by Zwave - this isn’t the case for scenes.