Re:

[Bertalan Fodor]

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

I came up with this solution:
       \once \override Voice.NoteHead #'print-function = 
#Text_interface::print
       \once \override Voice.NoteHead #'text = #(make-musicglyph-markup 
"noteheads.s1")
       c8

> 2 - How to obtain a key signature that have, simultaneously, the notes 
> b-flat a f-sharp?

See:
http://lilypond.org/doc/v2.6/input/regression/out-www/collated-files.html#key-signature-scordatura.ly

For advanced tricks looking at regression tests is always helpful.

Bert