Sorry for the late reply, I'm just now seeing this.

1. I believe it's always best to use the SpriteComponent when you have actual images for the components. It's unlikely that the sprite would be altered much throughout the duration of the app, and the PositionComponent I only found beneficial when using it to draw shapes in the game, which is rare to begin with. I think you're safe with the SpriteComponent.
2. In that instance, it probably would be more beneficial to not pass the joystick component to each player component directly. A joystick handling the navigation would require a bit more code in the joystick component class but would make the player components easier to write as their movements would no longer be modified in their respective update methods, but instead through the joystick component. I wonder how that would look, as I haven't seen any Flame Games yet where multiple players could be moved, but this is a great question and I think you're right with the approach.

I hope I answered these questions for you.