So like, when she gets resurrected, how will the people who knew her be affected? Aki has spent the last 5 years mourning, so when Koharu comes back looking like she hasn't aged a day, how is Aki going to mentally process that? Like, you can't just easily paper over that with memory manipulation. Even if you make her forget about Koharu's death, she's still going to be 6 years younger than Aki by the time she's alive again. Aki, who knows her primarily from being classmates in school. How is that gonna be rationalized? What about Koharu's parents? They had to bury their teenage daughter half a decade ago, but then she'll be back one day looking as if nothing happened? Mourning isn't something people just do privately, it affects major life decisions. So if you retroactively undo Koharu's death, all her loved one's actions since then won't make a whole lot of sense, even if you erase their memory of her dying. Without her death as context their remaining memories will still be nonsensical.
All this kinda makes me suspect that being resurrected is an empty promise that no cupid ever achieves.
The cupids do have to go somewhere eventually, otherwise their number would grow out of control. Unless there is a cap on new cupids, in which case not all people who die without having experienced love become cupids and Koharu just got selected somehow. Also, given enough time the cupids would get suspicious if nobody could graduate the program or if the graduates disappeared instead of being resurrected in the human world. Though I guess this hole could be filled with memory manipulation too.
One possibility to avoid being discovered after resurrection is that the resurrected materialize with a different appearance from before. Kanna only said "you will be back as a high schooler", but didn't specify further. The fact that their Japanese names have been redacted implies to me that their identities will stay officially dead. Providing the resurrected with a new identity might also be less work and lower impact than memory manipulation on massive scales.