OCA can of course make its own rules and criteria for curation/removal; I don't take issue with that.

The artist is accused of the broad offence of plagiarism, not merely violating OCA rules, and when that accusation is made I feel there's a certain burden of proof necessary, and this doesn't meet that criteria for me.

The difficulty with these cases is that they are extremely subjective and qualitative by nature. You cannot actually be certain that this artist specifically tried to duplicate that source, or even whether they had seen it before at all. Although I will admit it seems likely that it is a study (which, plagiarism or not, would certainly disqualify it according to OCA rules), the evidence is inconclusive within reason. The subject is a well-known character from an established universe which wears a specific outfit and has a specific look and feel. The pose is similar, yes, but the evidence ends there.

Combined with the progress shots, the artist's other works, and the low rewards of the post in question... I conclude that your effort here is not a productive use of rewards.