I came across this film that was produced by Hermann Schwenninger on euthanasia called "Existence without life". It wasn't officially released. The film was apparently lost but a reconstruction of it was made.
https://de.wikipedia.org/wiki/Dasein_ohne_Leben
Aktion T4, What is true and what is not?
Re: Aktion T4, What is true and what is not?
A video regarding t4 and 14f13.
If I were to guess why no t4 personnel were chosen to perform gassing that had experience with gassing, it would be because THERE WERE NONE.