First of all, do yourself a favor and get an Urtext...the Schirmer is not a particularly scholarly edition. I suppose it will do for casual playing but for serious study you should get a serious edition. It's worth the extra money. I also think this edition has many gratuitous and unnecessary fingerings. I prefer less so that I can choose my own and not have to cross out a lot of editorial fingerings that I disagree with.
Also keep in mind that the metronome was not around in Mozart's time, so any metronome markings are purely editorial and you should feel free to disregard it if need be.
As to fingerings, every hand is different and therefore different things are comfortable for different people. However, I think I can answer some of these questions for you.
In measure 2, last beat, why 4-2 for A-F# instead of 5-3?
Feel free to use 5-3. I usually try to avoid using 4 when possible in cases like these because it is weaker.
In measure 4, last beat, why 3-2 for G-F#? Why not 4-3 or 5-4? 4-3 or 5-4 set up an easy five-finger-based pattern for the next measures.
This is related to your question about measure 5. 3-2 is a particularly solid fingering, and because of the two note slurs present on the final beat of measure 4 and the first beat of measure 5, playing this passage as a completely connected four finger pattern would be incorrect. The two-note slurs allow you to make a break between measure 4 and 5. This answers your question as to why 3 for the first F#. It is a stronger finger, and it is allowed by the way the music is notated.
In measure 5, second beat, if 3 for F# and 2 for E, why 1 for the next E, which requires a hand shift? Seems awkward if one plays the movement fast. Same concept occurs in measure 6, first beat, with E-D.-D.
I agree. Since there are two-note slurs preceding the repeated note, it would be fine to just use a 2. The thumb seems ungainly here.
In measure 7, why alternate fingers on the Ds, which are slow quarter notes?
It makes sense to use different fingers for the staccato articulation IF you want, but it is not necessary. You can achieve it with the same finger. This is a matter of preference. Fingering should also serve the purpose of achieving a good tone. If you can do it with one finger, great, but if you feel the sound is better with different fingers, do that.
In measure 7, 1-3 is indicated for the interval A-C. What are good fingerings for the remaining two intervals plus the one in measure 8?
I think a better fingering would be 1-2 for the A-C, then 1-3 and 2-4. In this case I think it is more comfortable and stable than 1-3,2-4,3-5, especially because when you return to the G-B third, you will have a solid 1-3 fingering rather than the weak 2-4.
Every edition I've seen shows a curved line over all of measure 7 plus the first beat of measure 8. Nonetheless, in every recording I've heard, these notes are played staccato.
I don't know whose recordings you've listened to, but I've never heard this. Unless there are staccatos written in, don't play it staccato. I recommend Uchida's recording.
Is the curved line a phrase mark or a slur mark?
I may be wrong about this, but I'd say it is a slur mark because it does not carry over to the next measure. If it were a phrase, it would probably extend to the resolution in measure 8.
I do hope this helps. Good luck with your study of this piece!
Topic: Fingerings in Mozart K 283, I