I'm probably gonna be crucified for this given the relative lack of artsiness, but why not say 'landed amid the Zinnia's anthers and covered herself in pollen'.
Anthers being the rods which present the pollen of a plant. Stamen also works, as that's the whole male apparatus of a plant. More here:
A few alternatives:
She dived into the flower.
She dived into the zinnia's flower.
She dived into the petals.
She dived into the zinnia's center.
She dived into the pollen.
Add the rolling in pollen parts if needed.
I realize you want to differentiate between zinnia the plant and the actual flower. You don't want your ...
Putting scientific definition aside, "Flower head" works better since it's a personification. The human reader has no trouble associating the upper part of a body with the upper part of a flower.
On the contrary, "a flower's heart" is a little harder to imagine. Without further context, I would struggle to understand what you mean, exspecially since I'm ...
Your publisher will decide where to file your book, and it's largely a marketing decision. In the USA, books are often shunted into YA if they can possibly fit there because it's a healthy market. Also schools will often pick up or promote YA books --even ones that are a bit edgy --because they're trying to get kids to read. (Also for this reason, it may ...
Young adult vs adult fiction isn't about the age of the characters (though that usually does vary too), it's about the age of your readers.
If you're writing for adults, then write for adults and pitch your work that way to publishers, agents, and potential customers.
If you use a traditional publisher, they might want to classify your book as young adult, ...