Re:

[Bertalan Fodor]

>  
> 1 - How to substitute a notehead of an eigthnote to a notehead of a 
> halfnote?

I've found another (perhaps better), but also interesting (for me who 
now's going to understand Scheme and LilyPond better):
       \once \override Voice.NoteHead #'glyph-name-procedure = #(lambda 
(a b) "1" )

The number is from the end of "noteheads.s1", see the Feta font in the 
manual. The (a b) is the parameters that is needed for the 
glyph-name-procedure. I found by trial that it needs 2 parameters.

Bert