Inputting slash notes into a bar

Yes, you can do this. You might want to put an NC above the written pitches
just to make it certain that no chords should be improvised with the notes.

Here's a step by step for the notes in the bar.

1. Press N to start note input. This step is vital because the notehead
keystrokes won't work properly unless you do this.
2. Press Alt+Shift+3 for stemless slash noteheads.
3. Press F8 and then NumPad4 for quarter notes.
4. Type B to input a B. It should be a B4, if it isn't, use the up and down
arrows to correct it.
5. Press B two more times to enter the second and third quarters.
6. Press Alt+Shift+0 for round noteheads with stems.
7. Press NumPad3 for eighth notes.
8. Type in the two eighth note pitches.
9. Press N to stop note input.

For the chords

1. Select the first note and press Ctrl+K.
2. Type in the first chord and press space. Then repeat this step for the
second and third chords.
3. You should now be on the eighth note at quarter 4.
4. You now need a no chord instruction above the eighth note. You could just
type in "NC" as capital letters but Sibelius actually has a no chord symbol.
5. You get this by inputting a µ character. You can find it by opening the
word menu with the applications key and selecting µ in the table. It's in
the column with  dim at the top. So, as soon as the menu opens, press right
arrow until dim is spoken and then down arrow to µ.
6. Press Escape.