They work pretty much like normal tabs, meaning you can close them by middle clicking it, you can ctr+shift+T, ctr+tab, ctr+shift+tab, ctr+1, ctr+2...

you can close multiple tabs that are nested by closing the parent when its minimized.

I rarely collapse sub-trees; I rarely have enough tabs open for that to be needed. And I set the prefs so that if I close a parent, the first child takes over as the parent of the tree.