Both are useful, after all, and it’s not like there is a shortage of real estate
Is it possible to show both End and Length of a selection?
I don’t know of one, and that seemed odd to me, too.
I assume that you are referring to the Selection Toolbar (?)
That depends on how big your monitor is.
On a 1024 px wide display, there is just enough room when the time display is hh:mm:ss+samples
In response to user feedback, the next version of Audacity allows a more flexible choice of start time, end time, length of selection and centre of selection, but (along with the “Audio Position”) it still only has three time widgets so as to remain compatible with a broad range of display sizes. There may be more changes in future versions.