I’m not sure I understand this. I agree that mortals bring something special to the table, but I’ve always interpreted this as their dissatisfaction with the world that is (Which brings them closer to Melkor in this sense). Elves try to enhance and ‘heal’ the world that is, but humans tend to try to change it - hence the going ‘off script’ more than Elves do. But there is nothing essentialist about this (Exhibit A: Noldor). If they were bound to fate in the way you seem to be describing, then how does one make sense of eg. The Kinslaying(s), Turgon’s rejection of Ulmo’s message, to say nothing of the Rings of Power project. These all involve Noldor of course, but the Sindar are also capable of poor choices that lead to destruction (see Thingol).
It is not to say elves cannot do wrong. They obviously can. They cannot transcend the "natural order of things" - they work within that - intune with it - which is both their blessing and makes them vulnerable to beings who command the "natural order of things" - good or evil.
Humans are naturally "off", they disturb the "natural order of things" by their very being - this makes their own lives much harder but makes it also makes it harder for said "beings that command the natural order" to control them or forsee their deeds completely.
Humans can always rebel - even if seemingly completely enthralled (as long as their mortality has not been compromised - that is - as long as they have not lost their humanity and identity as mortals).
Humans are the agents of chaos to an extent (as they tend to want to force nature to follow their will - and are quite brutal about it),
but are also naturally able to "break the old mold" and make room for "the new that is to come" (to the horror of elves who are by their nature are agents of preservation).
[So simplifying - humans have the capability to "break" the natural order or act "outside of it" in a way that elves cannot - and do stuff completely new - not in the music at all - but lack the ability to work in harmony with it and "within it" to the extent that elves naturally do].
But the main issue is - if for other beings acting "out of song"
equals automatically falling from grace,
for humans this is
their natural behaviour and what
they are meant to do in-world
- if they do it wisely, within the bounds of good and reason and for the right purpose
- if they do not misuse their powers -
as any other being is also capable of misusing theirs.
Another beings who seem similar to humans in that regard seem to be dwarves (fitting - as dwarves were created "out of natural order" in the first place - and then retro-fitted in) - dwarves seem to be able to disrupt the natural order of things similarly to humans - but I think dwarves are not able to make room for something new
that transcends the natural order - they can just modify the natural order (often quite brutally) to accomodate them - but not to bring about something qualitatively "completely new".
I would argue that Elves and Humans share a lot of common ground, despite their different relationships between Hroa and FEA. Tolkien depicts them as both Incarnates with similar physiologies of their physical forms and they can have children together. They are capable of a similar range of emotions and physical needs (although Elves are hardier and have a different relationship with time). They are both subcreators by design. This is why I don’t need elves to be completely other-worldly for them to be convincing for me. They can be stronger, more experienced and skillful, more understanding of how the universe works, etc.. They can do things that seem magical to humans. And they can make assumptions about life and living that are strange to humans because of the differences in a mortal and immortal perspective.
The main difference between elves and humans is the the hope of the first ones lies in their past and the hope of the second ones lies in their future.
This makes the incarnation of Istari really interesting and I like the way this was portrayed in RoP. The Stranger is not accustomed to being incarnate and must learn to eat (and what hunger means), and he hurts himself badly when using powers at first. He is like a newborn baby at first and needs to learn who he is. When we leave him and Nori, he still doesn’t know his name or his mission, but is going on what evidence he has so far. In this season all he is trying to do is work out if he is a peril or not. In one sense he IS a peril - he is dangerous (something that Gandalf says when he comes back from the dead), but he is also altruistic ‘good’, as evidenced by his choice of actions. This teaches him something about himself. I don’t see any evidence that this is going ‘outside of the song’ in that sense.
I do not see the Stranger as a peril in RoP but just the Harfoots consistetly insisting on ignoring very basic safety rules. But that that is one of my problems with the show - what they intend to show and what they do end up showing does not seem to match at times.
By making the Istari resemble men the Valar show that they thought it the best form for the Istari to be able to perform their tasks. Sill, even if the Istari are made to look like men they are not Men and cannot claim the gifts of Men (Saruman is someone who seems to have forgotten that).
- So my point regarding RoP is: please do not make elves humans and do not make wizards humans. Otherwise the whole story stops making sense. Those other races are not simply humans with added on powers. There are specific gifts that are uniquely human and those make mortals suitable to do counter-fate-intuitive stuff that other people are just not well suited to do - like e.g. bringing the one ring to Mount Doom without Sauron ever noticing - and similar stuff.