Challenge II

Voices:
After looking at this score there doesn't seem to be anything that can't be done with 4 or fewer voices per staff. I may be wrong since I'm not expert on those sort of things, but I believe 4 voices is a limitation of the MusicXML structure. Someone will surely correct me if I'm wrong.
Expression markings and text:
There is nothing difficult about that, just type a staff text and move it into place.
Tempo changes:
There's not that many here, and only one in any given measure. What MuseScore DOES fail to handle are mid-measure tempo changes. I use these to achieve effects like Ritardando, accelerando, fermatas, pauses, and con licensia. Although the tempos can be placed, when the file is reloaded, they will not play back properly, and I suspect corruption of the file results in some cases.
Cadenzi:
These are easy enough to do. Just write them out ignoring bar lines, then count up the note values, copy the notes, Change the actual value of the measure containing the cadenza to something that fits (for example the last measure would be 23/16), then paste the notes back into that measure, cleaning up unneeded rests by making them invisible.
Tuplets:
7 and 8-tuplets work in 4/4 time. However the sephtuplet uses too many beams. There's a bug there.
Staff crossings:
Are entirely possible, although there is a bug the I have reported when manually beaming a group of notes that crosses between staffs. Also, I am unaware of a method to move a single note or selectable group of notes in one chord to another staff.
Editorial accidentals:
Musescore allows these, however it follows the normal convention of placing an accidental in parentheses before the note. I haven't seen any other instances of this type of editorial marking in a printed score.
On to what really glares at me as being a challenge for MuseScore:
Pedal indications are part of the music. If these continue to be discarded, playback is wrong as all notes will have the wrong durrations. Although Musescore does allow the pedal marks to be placed they are ignored when the score is played back. I'm sure also that many (most?) pianists would prefer the more descriptive line type pedal notation. The look of this can be achieved to some extent by placing lines with hooks at both ends, but the _^_ type of pedal point is still impossible.
Ties such as those in the left hand in measures 1 and 2 are impossible as they are not strictly (anal-retentive-ly) correct notation, even if they are common. You can get the look by using slurs but the sound would be all wrong even if the pedal were used.