The following works from the command line, but not from within Attractmode:
So I ran into some similar issues getting retro arch working too.
not sure if this helps, but here is a copy of my AM config. I use system specific configs in retroArch, and load the config as part of the args. The config references the core to load.
HTH.
# Generated by Attract-Mode 1.6.0
#
executable /usr/bin/retroarch
args -c /emu/retroarch/mupen64plus.cfg "[rompath][name][romext]"
rompath /emu/roms/Nintendo 64
romext .n64;.z64;.v64;.7z