Hello there, and no worries about the multiple questions.
We apologize for being so unclear about what is problematic and what isn’t, as we have received multiple questions about this. I hope this clears this up for you.

To start, contributors are MORE THAN allowed to apply if they create dark/mature themes like unhealthy relationships, trauma, and violence in their content, as long as they are appropriately tagged, like how Ao3 has its own filtering system. Contributors who create illegal content like incest, pedophilia, and non-con are NOT allowed to apply. We will make a list breaking this down in the carrd as soon as possible.

Our rules exist primarily due to the fact that a significant portion of our audience and contributors are underage and our zine is meant to be SFW. Hence, we are striving to curate content that is easy to read, enjoyable, and pleasant with a focus on Valkyrie, flower languages and lyrical themes. We understand your perspective and appreciate your input.

Lastly, the reason why side characters are preferably not allowed for submissions is that the focus of the zine is specifically meant to revolve around Shu and Mika only, but, as of reviewing, rereading, and revising, we can see how this may be difficult for our artists and writers, and we will change the rules to include more side characters.
If you are accepted into the zine, and your pitch needs to involve other side characters, that is okay by us, but Shu and Mika must be the main focus.