In many ways, everything about playing the piano is about creating illusions. The minute you play a note it begins to decay, yet we find ways of creating the illusion of phrasing. The instrument is percussive, yet we find ways to make it seem more vocal or orchestral.
Playing a group of legato notes that don't change pitch is also an illusion and we have to accept that we can't do the same thing that a wind player or a singer can do. If you want to play this in a legato fashion you simply need to put very little space between the notes and articulate the notes so one note is a smooth transition to the next. Of course, you can always use the pedal, but in many cases, including this one, it's not really appropriate. Generally speaking, I would take the long slur to indicate phrasing more than articulation — it is indicating that you should consider the first four bars as one musical idea to be played through rather than breaking it up into smaller pieces.
It will help you play this piece if you understand where it comes from. This is an arrangement of Non piu andrai, one of Figaro's arias, in Act One of the Marriage of Figaro. Mozart's tempo is marked Vivace nor Allegro. This tells you it should be quite quick. He doesn't use BPM. The metronome wasn't invented until the 19th century after his death. Additionally, the tempos indicate as much a feeling as speed. Vivace should be brisk — this is something you can accomplish at different speeds and the performer is given a fair amount of license to interpret.
You should also know that the rhythm has been altered quite a lot in this arrangement. Mozart's original calls for dotted-eight/sixteenth notes rather than straight eighth notes, which further give the piece a lilting feeling.
As for the last chord, you will probably play the second to last chord with 1 and 5 in the right hand. To go smoothly to final chord, you'll probably end up on 2 & 3 in the right hand.