While the series has pulled the rug, I doubt they would do that in Amane's case. The truck driver was a suprise victim, but it's clear that Shidou considers largely his brain dead patients and son as his victims and that is shown properly.
It's also clear that whoever she killed, she killed inside of a house (which she would need to enter), and that it's someone who would punishes her multiple timse.
After you cry, repent, and kneel, it’s now your turn to say that hopeless “I’m sorry”
"So there is no second time, I’ll give back the judgment that you gave to me"
Remember MY cries, MY repents, MY words of “I’m sorry” that I said to you?
There is five candiates, Gachata, Yuuri, Gozake, Riyone and Mrs Momose, as they are the only ones who Amane interacts with that punish her multiple times. But as easily we can dismiss four since they're characters.
While you could argue they people wearing the mascots are who she killed, it's very clear that these Mascots are representing Mrs Momose actions.
At the beginning of Purge March we see Amane leave the flag on the floor and mess up twirling Gozake's flag.
This transitions into her being water boarded for failing to comply with the teaching.
The teaching she messed up was "Tis ordained, thou shall discard vulgarity", otherwise known as in her head, Gozake's teaching, and so she depicts Gozake punishing her when it's her mother.
She messed up Gozake's teaching -> she gets waterboarded by her mother = Gozake waterboards her
We see this again later in Purge March and Magic
She helps the Cat, which goes against Riyone's teaching of "Tis ordained, thou shall follow thine destiny" since she is changing the Cat's destiny (to overcome their injury or die in trial) by helping them.
So, since an adult told her mother what see was doing, she gets tasered by her mother.
She went against up Riyone's teaching -> she gets tasered by her mother = Riyone tasers her
Even in Magic, Riyone is whom punishes her first, lining up with the events, and she could be recalling other punishments too.
The other two, Yuuri and Gachata, representing physical abuse and verbal abuse (or audio) are likely other ways Mrs Momose has punished Amane.
So if the only characters we see punishing Amane multiple times is the Mascots and her mother, and the Mascots are reflecting her mother's actions, then I think it's fair that Mrs Momose is her victim.
It also helps that all we see of the victim is their hand, and Mrs Momose throughout the entire Purge March MV is represented solely by her hand. It makes sense that the character who is always out of frame but their hand twice in the MV is the same as the one at the end of the MV.
BUT I do understand wanting to keep expectations open considering what has been revealed in T3 so far, however I'd be really really really suprised if it's not her. For a while I thought Amane had two victims since I thought her anniversary reflected her murder and she wasn't wearing those clothes in Purge March, so that who she killed in Magic and Purge March were different. It seems much more likely that Mrs Momose is her only victim but I think it's fair not to rule out the possiblity of more.