1. Nothing :)
2. We don't go into detail about selection manipulation on that page, but SurfaceFragment has some well documented methods, so it shouldn't be hard to add.
3. ve.init.target.mode ('visual' or 'wikitext'), however we have a general principle that tools should be at least visible (if not available) in all modes to maintain UI consistency.
4. See 2.

In T150081#2773797, @Esanders wrote:

1. Nothing :)

Nice! Though of course it seems a bit strange to have a visible delay to get the wikitext of your stuff, when you know that wikitext in advance anyway.

But ve.ui.MWWikitextCommandRegistry.js contains enough examples to answer most of the questions.

I'm not really sure if somebody who has the first contact with VE internals because he wants to port his gadget to NWE will be happy with https://doc.wikimedia.org/VisualEditor/master/#!/api/ve.dm.MWWikitextSurfaceFragment, so some examples would be nice, but for me this is probably enough, and T142523: Provide a second version of a beta modern wikitext editor integrated into the visual editor has "Successful proof of concept for a few gadgets" on the roadmap, which should help other developers.

I just created https://www.mediawiki.org/wiki/VisualEditor/Gadgets/Add_a_tool. I didn't yet link it anywhere, as I hope somebody can review it a bit first.

Uh, I more or less forgot about this.
I'd be really happy if someone could just have a look at https://www.mediawiki.org/wiki/VisualEditor/Gadgets/Creating_a_custom_command, whether I should just remove the "draft" tag. It should be read by someone else first, after all, I accidentally had unpaired tags in the code on https://www.mediawiki.org/wiki/VisualEditor/Gadgets/Add_a_tool for quite some time.