Two midi questions

[Alasdair McAndrew]

1. I have used
   
   \set Staff.midiInstrument = "viola"
   
   but the midi output still seems to contain a piano sound in it.  It's as though I can add an instrument sound to the midi file, but not turn off the default piano sound.  What's going on, and how can I fix it?
   
2. To change the midi tempo, I used
   
   \tempo 4=80 \once \override Score.MetronomeMark #'transparent = ##t
   
   which seems unnecessarily clumsy.   Wouldn't it be easier to have a "midiTempo" command, so that
   
   \midiTempo 4=80
   
   would have the same effect as the line above?
   

Thanks,
Alasdair