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You can find a list of hardware keyboard shortcuts at http://www.textasticapp.com/v5/manual/lessons/Whic...

I'm not aware of a shortcut to delete the current line.
I'll see what I can do.
Hrmm... Textastic is a sandboxed app and uses the standard NSDocument mechanisms. The Open panel and iCloud are completely handled by the operating system. Because Textastic is sandboxed, it has no access whatsoever to the open panel.

So I think that's probably something that needs to be fixed in Default Folder X.

Does this problem also happen in other apps like Apple's TextEdit?
You can just drag the file icon from Textastic's window title bar onto the other editor's dock icon. This will open the file in the other app while it is still open in Textastic.
In iOS 8 it will be possible for developers to create system-wide custom keyboards and distribute it through the App Store. I'm pretty sure someone will develop a Dvorak keyboard.