We are capable of either maximizing our lung capacity or squeezing our lungs by our posture. Bending the knees up towards the chest, for example, diminished the space in which the lungs expand. This is why there are specially-designed musician's chairs so that the rear of the seat does not tilt down. Also why, when not using these chairs, one is encouraged to sit on the edge so the knees drop downward more and keeps that space open. How we breathe is very important. And no matter who you are, you can tell whether your lungs are filling to the bottom or not, if you learn how to recognize it.

The didgeridoo requires it. I do not know about other non-standard instruments like the bansuri which the OP mentioned.

A lot of good in this answer. A tip about learning what a proper breath is: lay flat on your back on the floor and take in a breath. This will force air to the bottom of the lungs and will prevent the shoulders from raising and causing a shallow breath. (When upright, keeping the shoulders down helps good breathing.) I can support my breath a lot longer using my instrument because there is much more resistance to the air. When doing long tones, it is important to do them at different volume levels. Some wind instruments require circular breathing, which requires even more different techniques.

@Walter, the problem is that the computer program will never be able to truly replicate what real players do. The computer is unable to figure out foreground, background, etc. so you must put in these extremely detailed dynamic markings in order to get the computer to do it. Players understand when to bring out a line and when not to, so these extra markings are gratuitous and may even come across as frustratingly unreasonable and not understandable. How does a player know if their note between ppp and pp is not right?

Just to clarify, the LH half note should not be held two beats so that the C in the melody can be heard. In this situation where a note is held, but another plays "on top of" the same note, the melody note is the one that must be heard.

@Walter, why do you think 8 dynamic markings are insufficient? Do you play an instrument? I encourage you to ask yourself, if you were listening to a performer play your piece, how you would know whether or not that player achieved the minute middle between pp and p. As Lars said, the best thing you can do is work with some players you know in real life to see how they respond to your scores and play your pieces. The most important thing is that your music be reproducible by performers, if it is intended for live performance.

@Tim, I think in lieu of a written out chord sequence, a key is necessary. But for a song that does not fit the standard key/chord sequence expectations, something written out is an important courtesy.

@Tim, why can't you just be given a set of chord changes? Of course, it would be nicer if someone wrote them down, but with the chord changes given a key assignment is not necessary. My point above was that all a player needs is chord changes, not a key. A key is not necessary. You may find it nice, but it is not necessary.

@Tim, that will work only in songs that stick to one key or follow a standard chord progression. If the song is original and you just want the key, you are basically asking to make up your own progression instead of following what the composer chose.

@Dekkadeci, I agree that music with modal leanings often don't stick strictly to one mode (which I said above in the fact that keys can be so ambiguous and move back and forth.) So, whatever makes the music easiest to read is the best key signature, if one is used at all. My composition teacher said nowadays it is often better to not use one at all and just write in all altered notes.

@user45266, the key signature does nothing to clearly convey the tonal center of a key. It only is there for a short cut, grouping all or most of the accidentals in one place. If the song was in F major, all/most of the Bs would be flatted. Much easier to put a Bb in a key signature than to write a flat before every B. The key signature will not distinguish between C major, A minor, F lydian or whatever. That is the job of the composer who makes the chord progression and melody convey the key.

@user45266, this is not about knowing how to write the melody. The OP seems to have already written the song, as it ends on C. He is asking what key to call it. If it is written out for someone else, I already explained what needs to be done. If it is not written out, then the melody can be learned by ear and the player only needs the chords.

@Walter, that's great!

It is a necessary expense for serious composers. I balked at the price, too. But when I got $ at Christmas or my birthday, I bought it. I use it on every piece. Starting to memorize some of the information now...

It would be helpful to see the next few bars in the piece.