With Vuo 2.0 around the corner, it will include checkboxes & sliders, so it will be possible to use those as "Input Parameters" for exported apps too.
Right now you can already create your own checkboxes & sliders, but it takes more nodes VS the upcoming stock nodes for them (see joined composition Slider + Checkbox).

For text boxes, they're still marked as "chosen to be implemented", but not for the first Vuo 2.x. version.
You can also somehow make your own ones, but they really take quite a big amount of nodes, and need the custom Paradox nodes to be installed (link) (put the custom downloaded node from Github in your User Module Folder you can access from the Vuo editor Menubar > Tools > Open User Modules Folder & Relaunch Vuo) because the current Vuo version needs layer names for layers to be clickable (Vuo 2.0 will fix that if I'm right so it will already lower the amount of nodes needed).
(See joined composition Text Boxes)

And as said, since we cannot yet hide/close windows, right now you can show/hide the UI or parameters using a keyboard key (command key used in bot joined compositions).

Yes, looking forward to it too, and in exported screensavers too.

Yes, OSC is a way to go, one can also create a separate Vuo window for the controls by duplicating "Render __ to Window", but to be able to hide windows or closing them, we'll need : Nodes for showing/hiding windows, and to enable the window's close button

With Kineme the team built upon QC, here, they built it from the ground, still pretty young software, so features are added over time, still missing some features, but already totally loving Vuo fro what it offers ;)