This is something that hasn't been done before in game, but as for how it would work:
If you wish to have Maria die as a god, return as a shinki, and have her in-game death be the death that her future god would see when naming her, you can do that.
Usually, when gods have switched to shinki by picking a canon "death" point, they would then be able to safely hear things about their time in game, as they would be coming straight from a canon "death," and therefore their personal timeline would be different from their game timeline. For a character who is apped as a god and becomes a shinki by choosing a canon "death", the character can't get back in-game memories of their time as a god through regains of any kind, but can safely be told about things they did and people they knew as a god.
If you use an in-game death, Maria's time in game would then be a part of her "life," so it would be dangerous for her, as a shinki, to be told any information about her past as a god.
You would not be able to use the Reverse Death regain for a character going from god to shinki, regardless of whether you choose an in-game death or a canon death point for her. That regain is specifically for regaining memories lost through the death mechanic due to an in-game death. If she's coming back as a shinki, those memories are lost because she is now a shinki, not because of the in-game death.
However, if you use an in-game death, you would be able to regain memories of her time in game through other memory regain mechanics. For example, she could remember close CR through a Single Person regain, or remember important events with one of the Random Memory regains.
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If you wish to have Maria die as a god, return as a shinki, and have her in-game death be the death that her future god would see when naming her, you can do that.
Usually, when gods have switched to shinki by picking a canon "death" point, they would then be able to safely hear things about their time in game, as they would be coming straight from a canon "death," and therefore their personal timeline would be different from their game timeline. For a character who is apped as a god and becomes a shinki by choosing a canon "death", the character can't get back in-game memories of their time as a god through regains of any kind, but can safely be told about things they did and people they knew as a god.
If you use an in-game death, Maria's time in game would then be a part of her "life," so it would be dangerous for her, as a shinki, to be told any information about her past as a god.
You would not be able to use the Reverse Death regain for a character going from god to shinki, regardless of whether you choose an in-game death or a canon death point for her. That regain is specifically for regaining memories lost through the death mechanic due to an in-game death. If she's coming back as a shinki, those memories are lost because she is now a shinki, not because of the in-game death.
However, if you use an in-game death, you would be able to regain memories of her time in game through other memory regain mechanics. For example, she could remember close CR through a Single Person regain, or remember important events with one of the Random Memory regains.