I have released an updated (but still incomplete alpha version) of TruePreview, TruePreview 1.8a6. This version only adds support for Mountain Lion, OS X 10.8.4 (Mail 6.5) but is functionally equivalent to the previous version.
The following is still true in this version: Due to changes in Apple's code, the reply, forward, and scroll/click options are unimplemented and disabled in this version. I will (hopefully) add them back when I figure out how to make them work. So, this version only contains the delay (immediate, never, X seconds) and "Viewing in a separate window" functionality. NOTE: This version is only for Lion and Mountain Lion Mail 5.0-6.x. If you have an older version, please continue to use version 1.7.
I very much appreciate all of you who have contacted me with words of support and encouragement. I am also extremely thankful for all of you who have made a donation to support the development of TruePreview!