Re: The stemlength of the Longa cannot be changed.

> % The stem length of the longa cannot be changed. It is too short
> % for the Baroque NoteHead style.  Longas are normally used as the
> % final note in Renaissance music.
> 
> \version "2.26.0"
> 
> \relative c' {
>   \time 9/2 c2 c\longa |
>   \override Stem.length = #2 c2 c\longa |
>   \override NoteHead.style = #'baroque c2 c\longa |
>   \override Stem.length = #6 c2 c\longa |
> }

Please open an issue in our tracker at

  https://gitlab.com/lilypond/lilypond/-/work_items

and provide scans of printed scores (ideally, both from the Baroque
era and modern Urtext edtions) that demonstrate how such notes should
look like.  Right now, the neomensural types are used, which is not
ideal.

BTW, it is by design that the stem length of longae cannot be changed.
If you look closely, the stems don't have a standard shape.


    Werner