There is an abundance of information regarding W.A. Mozart and Symphony Number 41. Here are a few websites regarding Symphony Number 41:
Burgess Hill Symphony Orchestra program notes
The Kennedy Center
I also employ the Cambridge Music Handbook on Symphony Number 41. I have had success using handbooks in this series on the previous prescribed works.
As there is more information on this work than on El Salón Mexicó, the IB Music class should discuss at length what information is pertinent and what information is superfluous in regards to the examination.
In the Mozart, focus should be placed on the ‘classical’ elements. With El Salón Mexicó, I believe more focus should be placed on the ‘World Music’ and non-western elements.
A Mozart ‘understanding’ for a student (I believe) should include such things as (and this is not exhaustive) structural elements (location), form (sonata), instrumentation, etc. – the more important point is that students must be able to relate their discussions back to Classical music and how the components of the piece make the piece Classical.
How is the Classical style (or Mozart’s style) exemplified in the piece?
That depth of knowledge would be an excellent Mozart ‘understanding’.