Yes, I think this is unwise. In my view, it is a model of salvation that is widely held because it is
superficially plausible, but would require extensive and deep changes to our understanding of God, ourselves, and the spiritual Laws of Nature (so to speak) if it were actually truly possible. If you're willing and ready to explore the implications of "why can't God just forgive sins without propitiation?" ... proceed very cautiously at your own risk. Here Be Dragons.

Reiyen wrote:
I have a character who is from another world. There, God has never and will never incarnate.
By "another world," do you mean "another universe" (
a la Narnia), or "another planet"?
Reiyen wrote:
Would it make any difference if I now divulge that my redeemed character is not a human, but a spiritual being, as old as any other non-God spirit?
As you've stated your idea so far, no. If the category of beings to which he belongs is established as having the ability to be brought back to life, and you establish that the penalty for crimes among them is death plus restitution-labor,
that is so far not entirely unreasonable (cf. Coriakin). If at some point---preferably
before completing his labor, IMO, for various reasons---God imputes righteousness to him, that's still somewhat problematic, but less so than having the forgiveness come as a result of his own works.
OTOH, if anything making him a purely spiritual being may introduce a whole new set of problems: we human beings are made so that, once regenerated, we
can repent, but an angel, demon, or other purely spiritual being, not being (as Lewis puts it in
The Screwtape Letters) "amphibians," doesn't seem to have that capacity. (On the gripping hand, the best reasoning for that that I can come up with is inductive, so introducing a category of spiritual beings capable of repentance would complicate cosmology but wouldn't contradict anything in Scripture as far as I can see.)
Reiyen wrote:
an exploration of other ideas (the meaning of obedience, for example)
What about obedience are you thinking that this would illustrate? Because my understanding is that apart from Christ, without the regenerating work of the Holy Spirit, it is
impossible for a human being to fully and willingly obey God. If, once God makes him righteous (and changes his nature?), your character wants to and finds himself able to obey God's commands, that would be a worthy theme, but that feels like rather the reverse of what you've described.