> I would like to ask your help geting back the added line /or extra staff line (I couldn't find the appropirate name for it either)
> So under the common 5 lines in the staff - somehow we lost the automatic cross line for c, or h,
I'm no expert w/r to gregorian in lilypond but my guess is that the culprit is the \melisma music function which in turn invokes the \neume music function which overrides the NoteHead.stencil.
I'd guess that you'd have to (re-)add a Ledger_line_engraver but I'm not knowledgeable enough to do so.
May be it is a bug in the modernGregorian style?!
Kind regards,
Michael