From measures 1 to the first beat (A) of measure 14 we hear 2 voices, a soprano+alto duet. If you play JUST the 2 voices to simulate singing, the phrases in each voice need to be connected as an implicit legato; sometimes it's quarter note duration (like in measures 1, 3, 5), sometimes it's 8th note duration (like in measures 2, 4, 6), but always together in duration separated by a 3rd or 6th interval.
The rest are fillers for harmonic, rhythmic movement, flourish, and cadence functions. For example:
- the repeated 8th notes A in measures 1-2 are fillers for a sense of rhythmic movement
- the doubling of the soprano voice in the upper register (ex. where the left hand crosses over in measures 4-5 including the cute grace notes in measure 9) are flourishes to add character
- the notes in the bass clef are mostly cadence highlights (such as measures 3-4, 7-8, 11).
Mozart is creative in that in measure 10 he transposed the alto voice to the lower register while the usual 8th note rhythmic filler stops into the half note E to let the soprano and Alto have the focus as well as preparing us for the cadence in measure 11.
Within this passage context, it's then clear that we have to analyze measure 11 as follows:
- First voice (implicit quarter duration): E2, C#2, B which resolves to A in the next measure
- Second voice (implicit quarter duration): C# (in bass clef), A1, G#1 which also resolves to A in the next measure
- Cadence highlight: E in beats 2 and 3 in the bass clef, resolving to A in the next measure
- Rhythmic filler (off-beat 8th notes): A1, E1, E1
I think in measure 11 the alto is notated in quarter notes because the voices at this point have quarter duration, and Mozart (or the editor) helps us identify A1 and G#1 as the 2nd voice continuing the first beat C# in the bass clef. Since the character of the 2 voices in this passage moves together in duration as well as in melodic contour, the alto needs to be heard with the same duration as the soprano. Although in the notation there is a common beam for the 8th notes, it's very important to identify and to play the off-beat 8th notes A1, E1, and E1 as filler, not as part of the soprano.
A side note about the primacy of highlighting voices vs. actual notation
Highlighting the voices is more important than how long you actually hold the notes. In the first video we can hear a good rendition of the 2 voices performed according to the analysis above. Even though in measure 11 she plays the 2 voices about 1/8 duration, we hear it as a connected phrase (each note 1/4 duration). It's the effect that counts, after taking into account the room's acoustics and the decay curve of a note of a specific piano (remember, piano is by nature a percussive instrument), so pianists need to constantly adjust the actual length of holding the notes through feedback mechanism from what we hear.
This is especially true in Mozart's repeated notes (such as the left hand in the first few measures in Sonata Facile) where despite all are 8th notes, no artist would play those notes exactly the same duration. Otherwise, it will sound robotic as the "Play" button in MuseScore will demonstrate. I bet you that the MIDI duration values of Ms. Mitsuko Uchida's playing it will be very different for each note since she's holding the Cs and Ds much longer than the other 8th notes to highlight the bass line. The same goes for the right hand scale runs in measures 5 to 10 as Lang lang demonstrates various options we can play it.