For the shadow, I'm not refereing to some huge black shadow like the Mirage F-1 from Kirk Olsson for example. This one is exagerated, I mean, it looks good but weird when you're inverted, just like you said.
I was refering just to some very slighlty blurry dark aera, only very near to the base of those elements. In this way, it wouldn't be shocking when inverted, and it would look a bit better in normal flight. But again, this is something that must be tested several times prior to finding the correct balance

PS: I think the default repaint of the Piper Cub has some painted shadow like that, under the main wing. It's a bit too strong, but still the final effect is quite cool.