[EDIT] Thanks for all the unbelievably quick responses, but it turns out I was completely mistaken: The problem were not the out-of-range notes, but that they were overlapping with the lyrics of the line above, so attempting to select a note always selected the lyrics element along with it. So please ignore everything below.
Do I understand correctly that out-of-range notes are somehow partially blocked from editing? I have notes showing up in red, and they somehow behave different. I can do some edit operations, but others do not work. Why?!? I understand the value of a visual indicator when the range is exceeded, but why should that interfere with regular editing operations?
This is made even worse by the fact that Dorico Elements appears to offer not means to adjust the range of an instrument. I am doing my first steps on a simple Barbershop arrangement and happen to have singers with unusual ranges. Does that make me a professional musician?!?