Import musicXML - instrument assignment

Hello everyone,

I have the following project:
- Write a very reduced score, with few staves, indicating when instruments take up or leave the melodic line using chord symbols (example: +Vl on a note to say that, at that moment, the violin is added to the texture).
- Export to musicxml
- Using a python script, generate a musicxml file containing a staff for each instrument involved.
- Import this musicxml into musescore.

Everything works fine, except for the instruments assigned by default to each track, even if you modify the instrument.xml file.
For example, the violin(section) is automatically assigned to the musesound instrument "violin 2".
It doesn't seem possible to choose between "Trumpet" and "Trumpets".
The flute is assigned to the "MS Basic flute", not to a musesound...

In short, is there any way of choosing precisely which musesound sound is assigned to the "instrument-sound" tag in musicxml?
If not, is there a way around the problem? (In the worst case, I "only" have to modify the mscx directly, but well...)

Thanks in advance