…this might not solve the separate parts problem, but the grille is not too hard to paint within the bumper unit…

…remember that Satellites got bright argent paint in the grille, and Road Runners, GTX’s and Satellite Sebring Plus cars got a two tone grille (flat black for most of it, lower edge is dark argent).

Rather than using photo etch, if it were me, I would try to grind out the back of the grille and open it up (carefully). The photo etch grilles look too “flat” to me. The grilles in 1:1 have depth between the vents that the kit pieces do capture pretty well.

Just my $0.02.