From the research I have done, it seems that Nero 220.127.116.11, Nero 18.104.22.168a, Nero 22.214.171.124c, and Nero 126.96.36.199d are all almost the same (including version number from "About"), differing only in a DVD codec. Their installers also differ in what they may install: Nero 188.8.131.52c and Nero 184.108.40.206d come bundled with the third-party Ask.com toolbar and The Weather Channel Desktop application. Installing the third-party software is optional. Nero 220.127.116.11d cannot be installed as a trial version. Nero 18.104.22.168a seems to be the last version of Nero 6 that had installers without bundled third-party (to Nero) software.
Nero 22.214.171.124, Nero 126.96.36.199a, Nero 188.8.131.52c, and Nero 184.108.40.206d all have been officially released by Ahead or Nero (while it is unclear whether Nero 220.127.116.11d was later removed from the Nero website and replaced by Nero 18.104.22.168c).
Calling Nero Customer Service on different consistently yields no useful result. One holds for a while in what appears to be a false queue while music plays, then the call is dropped at the Nero end before speaking to a Nero agent (after the queue has already reached one).
Nero 22.214.171.124d seems to be the most up-to-date version. Note This does not necessarily mean that Nero 126.96.36.199d was the most up-to-date version sold on discs in retail channels.