To clarify on backends.cfg: If it shows jzintv="dispmanx" the emulator call gets these environment variables passed in: SDL1_VIDEODRIVER=dispmanx SDL_DISPMANX_WIDTH=1600 SDL_DISPMANX_HEIGHT=1200 /opt/retropie/emulators/jzintv/bin/jzintv ... (but has no effect on the emulator), if it is absent or configured to SDL2 KMS in _Configurations/Tools _ (results in jzintv="default" in backends.cfg) no env variables are prefixed to the emulator call. So long story short: No need to explicitly set something in backends.cfg.
On readline: You are right, if it is disabled in the build, a simplified implementation will be used. So I will leave libreadline-dev in the dependencies.
Will file the two PRs shortly, will use $__archive_url.