@Jack Yoff well like the vice commander stated the compound in the cup was found on the mariette dress not on seravi so there's no logical explanation for her being the culprit hence why using that ability for didn't make sense i guess..
This is how the whole suspect stuff eventually happens after mariette death, he called her a murderer after the incident that was then, but he still doubted Mariette committed suicide and his first suspect was still seravi but the facts went against it added the fact that she's a stalker didn't make her look less innocent in the prince's perspective till the reincarnator took possession of her body things changed. Hence why he showed her his memories and asked "who do you think the culprit is?" And she states "for someone who suspects me his demeanour is rather gentle" implying he already discarded her being the suspect after she reincarnated
This is how the whole suspect stuff eventually happens after mariette death, he called her a murderer after the incident that was then, but he still doubted Mariette committed suicide and his first suspect was still seravi but the facts went against it added the fact that she's a stalker didn't make her look less innocent in the prince's perspective till the reincarnator took possession of her body things changed. Hence why he showed her his memories and asked "who do you think the culprit is?" And she states "for someone who suspects me his demeanour is rather gentle" implying he already discarded her being the suspect after she reincarnated