1) Yes. The Kindles read any non-DRM Mobipocket files, and calibre will convert almost anything into Mobipocket format.
2) No difference between KK3 and Kindle Touch. The Paperwhite's display is slightly higher resolution, as well as having the frontlight.
3) The TTS is better than nothing, but it isn't brilliant as a guide to English pronunciation, especially for unusual words.
It certainly works for Mobipocket books. I think it would work on plain text, but easy to convert to MObipocket if not.