The editions vary in their metronome markings and I believe none of them. Your tempo will largely depend upon the state of your technique. To the second question my reply is that Chopin has composed "music" which—as you know—represents thoughts only in a musical sense, otherwise it deals with purely psychic processes, moods, etc. The humour of this Impromptu is mainly an amiable, ingratiating one, here and there slightly tinged with a sweet melancholy. It should not be played too fast, for it easily loses this latter attribute and then sounds like a Czerny exercise. A moderate tempo will also tend to bring out the many charming harmonic turns which, in too quick a tempo, are likely to be lost.
Chopin's Barcarolle
In Chopin's Barcarolle there is a number of trills preceded by grace notes. Are they to be executed according to Philipp Emmanuel Bach's rule, so that the grace notes take their time from the note that follows them?
Philipp Emmanuel Bach's rule is a safe one to follow, but do not confound a rule with a law. If you have reached that plane on which an attempt at the Barcarolle by Chopin is rational, you must feel that your individual taste will not lead you too far astray even if it should prompt you occasionally to depart from the rule.
Chopin's Works for a Popular Concert
What works of Chopin would you suggest for a popular concert programme?
Nocturne, Opus 27, No. 2; Fantasy Impromptu, Opus 66; Scherzo, Opus 31; Berceuse, Opus 57; Valse, Opus 64, No. 2; Polonaise, Opus 26, No. 1; Chants Polonais (in Liszt's transcription).
Taking Liberties with the Tempo
In playing Chopin may one take liberties with the tempo and play different parts of the same mazurka or nocturne in various degrees of tempo?
Undoubtedly. But the extent of such liberties depends upon your æsthetic training. In principle your question admits of an affirmative reply, but a specific answer is impossible without an acquaintance with your musical status. I recommend that you be very cautious about "taking liberties"; without, however, ceasing altogether to follow the promptings of your good taste here and there. There is such a thing as "artistic conscience"; consult it always before taking a liberty with the tempo.