All Well in Theory

Not all pieces begin with a section to be repeated (though plenty may do); but each stave will need ~ in this order ~ a clef, a key signature (even if that happens to be blank, as for C major / A minor) and a time signature (unless the piece is so metreless as not to need one)

The third step is the Mediant (as in Answer 2); the Submediant ~ perhaps a trifle perversely ~ is the note next above the Dominant, so it sits higher within the scale than does the Mediant itself. (These would be A and E respectively, within a plain scale of C major)

Vibrato, or tremulation, is usually at the performer's discretion.
Informally, session musicians often also use a wavy line (usually with increasing 'amplitude') to remind them of a ritenuto or slowing-down, particularly if this will involve watching others to coordinate it with them

The convention is to start with the upper woodwinds (typically, piccolo) and so down to the bassoon/s; then the brass, then the percussion (tuned percussion usually lowest within the section, e.g. harp &/or kettledrums), then the strings. The 1st violin stave may thus well be only a few lines off the bottom of a page with 25 or even 30 lines

From any B upwards to any next F, note-wise, is a fifth, but this consists of 7 semitones rather than the 8 of a Perfect Fifth, so it is described as diminished. The inverse interval (F natural up to B natural) is a diminished fourth. There is no such thing as a major or minor fifth

Answer 2 is a dynamic instruction ('louden lots', as the Australian Percy Grainger would alternatively have put it, since he sedulously refused to apply Italian terminology to British folksongs or indeed anything else). The others are all more to do with pace and style: (1) Moving along gently and gracefully, (3) Moving slightly more slowly than (1) and in a singing style; (4) Really quite broadly, and with a heaviness of tone (e.g. in the solemn manner of a funeral march).
... And, of course, Answer 2 was the only one which happened not to begin with the letter A!

Don't be fooled by the 'semi-', which may have suggested half the length of something: this is 'half of' something ... but of a breve, which lasts a full 8 notes, so the semibreve is still worth 4 beats. A minim is half that length, a crotchet half that again, and a dotted quaver is 3/4 of a crotchet (half, plus half as much again for the dot)

Leger lines are temporary additions to the stave, e.g. to show Middle C relative to the treble or bass staves. Answer 1 is a false-etymological red-herring ('line' accents are more as in Answer 4), and what Answer 3 refers to is correctly called Beaming, as in the yoking of three or four quavers or semis that fall within the same main crotchet beat

'Compound' indicates that the sub-beat is not divisible by 2 (which would be 'simple time'), and 'triple' shows three beats to the bar; so 3 x 3 = 9. This is the effective beat structure for pieces as diverse as Bach's famous cantata movement Jesu, Joy of Man's Desiring ... or a jazz-waltz ('swung' three-in-a-bar)!

The tonic minor has the same key signature as the major key that starts three semitones higher; this happens not to be because of these semitones (of which there are three, but we are counting upwards), but we add three flats.
For example, C minor, B♭, E♭ and A♭ share a key signature with E♭ major, though often music in C minor will have its B's re-naturalised to make a keener leading-note, just a semitone below the tonic