Regarding your first question, there’s no easy solution since each mode uses a different backend render and there’s little-to-no interoperability between the two (FX2D draws into javafx.scene.canvas.GraphicsContext; P3D uses JOGL).
With your second question, I understand it to mean using a JavaFX application (not necessarily a Processing sketch) to launch a P3D sketch. This is fully possible. In fact it’s possible to use a FX2D sketch to launch a P3D sketch.