It’s impossible to answer your first two questions without more context.
The final question: It’s a breve (2 x semibreve) - 9 on your keyboard (To get this round shape, see Engraving Options>Notes>Noteheads)
Note that in the original score bar 58 goes into open meter (unmarked) for the rest of the page – not 6/4. I would write the whole Solo part first, then add the notes of the other parts, and hide unwanted rests in between. You can still tie notes over rests (hidden or not) by selecting both notes first.
I would write the groups of eighth notes in the Solo as triplets, and hide the 3’s, since they read like triplets, and it would make it easier to align the other parts. The Bassoon part will need a separate cue staff for the solo in this passage.
The trickiest part of this will be aligning the Bassoon notes with the grace notes in the Solo part. This will require the secondary note spacing handles in engrave mode in both the score and the Bassoon part.