The problem is caused by a somewhat unusual way the airplane exterior in the iFly 737 behaves. Even if you don't have GSX installed, if you go very close with the external camera (but still slightly about the fuselage root), around the airplane center, the whole exterior disappears. And, it never shows in VC. Without the exterior visible, no Seated Passengers will show up.
We asked clarifications to iFly, because it's the only plane that behaves like that. It can possibly be fixed by changing this line in the airplane model.cfg ( for all liveries ) from this
withInterior_showExterior=false
to this:
withInterior_showExterior=true
However, I have no idea of the possible side effects, that's why we asked iFly about it, and wanted to wait for their ideas before releasing official support for it.
Pink passengers, instead, are completely unrelated to this, the manual has a chapter about that on Page 98, no use repeating it here, since it lists all possible reasons for missing texture.