Hi,
in answer to the problems you are having converting .mkv to DVD, this is a bug in ConvertXtoDVD.
You aren't doing anything incorrectly. A newer format .mkv file came out recently, and it is not being converted correctly. See:
club.cdfreaks.com/showthread.php?t=221935and
forums.vso-software.fr/post8571.html You can wait for vso to release a new version, and hopefully fix the issue, or you can demux/remux the .mkv files and then convert. Use Video Help
www.videohelp.com and google for advice on how to do this.
As an aside, when it comes to internal subtitle files in either .mkv or .ogg, ConvertX cannot see them, so you need to rip and use them externally. Again look up demuxing for information how to do this.
Hope I answered you questions.
mscottj