Kevin,
You're correct. Camera angles create a thing called "foreshortening."
Has to do with perspective and the angle of vision.
There was, years ago, a whole article about tracing the fuselage side of an aircraft and how to deal with this.
Obviously if a 3-view wasn't available or plans.
With CAD programs like mine, I could do a draw then actually grab the end of the graphic and "pull" or stretch it until it looks correct or just better.
No matter what mouth you draw, you will still need a mask for each color or, a simple way, would be to use brushes and paint the thing on white vinyl, then apply the vinyl.
Hand painting the thing on the model could be cumbersome whereas the vinyl would be flat and easer to rotate if necessary.
Easiest way is to make the thing using colored vinyl.
You have a really nice model happening. Kudos!
Charles