The problem with creation is the inherent liability of the creature. It is not the imperfection or fault of the Creator. Created creatures, created with intelligence, have a certain liability. That liability comes to light with the existence of a certain kind of knowledge, inevitably inherent in creation itself.
Thus the warning to mankind not to partake of the tree of knowledge of good and evil, as a means of life - as a means of gaining or sustaining the kind of life that beings with spirits or spiritual souls may partake of by virtue of that creation within them - the capacity of spiritual thought and contemplation of spiritual concept.
Thus the liability in creation, already foreseen, already foreknown, already a glaring problem if there is to be a creation at all, which involves intelligent beings. And what would be the purpose (other than purely cosmetic) of a creation which did not contain such ? Where would be the benefit to the Creator ?
So it was 'in hope' that the creation was ventured upon by the almighty. Else, would he not have ventured. To make a creation which would, inevitably and unsustainably, sink into a torrid state, would be (to say the least, if one may, in reverence, without even hinting at the possible consequences) 'unethical'.
No, it was 'in hope' that a creation was made which would, inevitably be 'subject to vanity'. It had to be, this inevitable sinking. Yet provision was already made in the mind and heart and purpose and intent of the Creator.
Thus the wording of Romans 8:20-21 (TR undisputed)
τη γαρ ματαιοτητι η κτισις υπεταγη ουχ εκουσα αλλα δια τον υποταξαντα επ ελπιδι
... for to vanity the creation was subjected not willingly but by reason of the subjecting in hope [EGNT but altered]
I have quoted the Englishman's Greek New Testament interlinear literal translation (1877) except I have said 'the subjecting' to properly (as the question suggests) express the article and the participle (1) rather than introduce an agent, contrary to the text.
No agent is mentioned. There is no ambiguity about blame, whose fault it was. There is a statement regarding a creation that was made and that became (immediately) subject.
The creation was made. It immediately became subject to vanity.
But the creative act was in hope of a better creation, for the first was merely a precursor. A means to an end.
Immediately the subjection, then immediately a promise of hope. A seed to come, not of the first humanity : of woman but not man. A seed that would - from above - bruise the head of the serpent (the cause of all the trouble) and should reign in a rearrangement, God manifest in flesh ruling in the throne of the Majesty in the heavens.
'The subjecting' was 'in hope'.
It had to be, thus. By divine foreknowledge and by divine wisdom, all was foreseen.
And provision was already made.
The Greek article derives from the demonstrative pronoun (that) according to Daniel B Wallace in his book 'Beyond the Basics' and it could be so translated here :
For the earnest expectation of the creation ... awaits ...
for to vanity the creation was subject ;
not willingly, but by reason of that subjection [being] in hope ;
that ... the creation shall be freed ...
In our own English idiom, the demonstrative pronoun (that) is referring back to the matter of subjection. It is not referring, nor is the participle, to an agency which brings about the subjection. It only refers back to the former statement about subjection.
(1) τον υποταξαντα . . . . . . article plus acc.sing.masc.part.aor.1 act.
[Bagster's Analytical Greek Lexicon 1870]