Just tap the Save button in the top menu bar. It will save the wiki file to your USB drive (www/itw.html).
Tap the Menu button in the top menu bar and select New Tiddler from there. A new empty Tiddler (TiddlyWiki speaking of a "Wiki Page") appears. Just enter a title, text and a couple of tags and you're done. Tap the done button in the edit menu (blue coloured).
Open the Tiddler in question, edit it and add the following tag to it: Home. Next time you visit the start screen (by using the menu or after saving and re-starting TiddlyWiki) your Tiddler will appear there.
Open the Tiddler in question, edit it and add the following tag to it: menu. Now a link to the Tiddler will be shown in the menu.
That's very easy: open the app menu and choose Restore. A list of backup copies will appear, each backup file has the timestamp of its creation in the filename. Tap on the copy you want to use to restore. Then, tap the Save button in the top menu and it will be your production TiddlyWiki.
There are various ways to do it. You can use the ImportTiddlers plugin in TiddlyWiki (reachable via Menu => Commands => ImportTiddlers) and import from a remote URL whatever you need. Or, download the itw.html TiddlyWiki file to your desktop, open it in your browser and add the Tiddlers you need from there. After saving and uploading to your phone you'll have your stuff right there.
Depends. It will not be delivered with pre-installed GTD plugins of any kind ever. It would become too large (thus slow) and GTD doesn't just fit to the webOS screen dimensions. However, you may add this to your wiki yourself, there are lots of resources on the web (look for "Monkey GTD" for example). But be warned: you may have to invest a considerable amount of effort to modify it so that it fits to webOS. If you ever made it, feel free to drop me a message, so I can consider to add it to the distribution.
Indeed! You may consider to donate to the developer (me!): Donate via PayPal.