> I find the support for DNG files is very limited and Cerious ignores requests for bug fixes. DNG files created by Adobe Lightroom 5 are displayed incorrectly.
The actual image data in a DNG file is never edited. What Lightroom does (well, can do) is remember the editing steps that you applied and save those (in Adobe's proprietary format) to the metadata section of your DNG file. Only Adobe software can re-apply those edits to make your DNG file look like it was when you finished editing it in Lightroom.
If other, non-Adobe, programs do (seem to) show the edited version, it's because you made Lightroom update the embedded JPG preview that's contained in the DNG file, and those other programs show you that JPG preview instead of the actual raw image (which, again, remains unchanged from the moment that you took the photo, and which is what ThumbsPlus shows you).
Finally, note that the Digicam plugin actually consists of two plug-ins, Digicam (plug_digicam.tpp) and DCRaw (plug_digiraw.tpp), and you can use either to develop raw files. Both also have various configuration options that you can set through Options | Digicam Plug-in and Options | DCRaw Plug-in, respectively.