Sadly not yet, at least not with a built-in function. Loved that in QC back then, but it stopped working with QC a while back too at some macOS update.
Just created a feature request for that since I'd love that too (have to wait until the team reviews it and opens it up for voting before voting is possible) : A UI for storing / editing published ports (save prefs) for screensavers
The only way I see now for people to change in image would be to go to the screensaver package, right-click to show package content, navigate to the image in the ressources folder and replace it with theirs I guess.
You just need to think Vuo as pipes. Events stream down the chain as a push system only (for now).
This means every node that should execute should receive an event, otherwise it won't fire.
The amount of nodes it depends for what.
For some cases maybe, but for some others I don't think.
There is not yet an integrated particle system, it's on the feature request you can vote on : Particle emitter node.
It has quite many votes so I expect it to pop up sonner than later (all opened for voting requests are here : Feature Requests.)
I just uploaded a quite dirty and heavy method using stock nodes for limited particles. Of course here a lot of math is involved, and the upcoming dedicated node will make that way easier. Someone made a particle effect node a while ago, but for Vuo 1.2.8, and he did not update it for Vuo 2. Was very powerful and simple.
Link to my stock nodes particles comps : Stock Node Particles.
If you have other specific questions or need help on a specific part of a composition, ask, all questions usually are answered here.