I would highly recommend not using Retroarch for Gamecube or Wii, the dolphin core really is bad, worse perforformance and emulation, less features, more incompatabilities, honestly just avoid it and use the latests nighly standaloane version (not the stable version from like 5 years ago). As for your actual issue id suggest you have bad roms if they fail to launch in both Retroarch and the standalone version, the fact that you get a bunch of errors at launch would bare thios out too. Dolphin does do fulscreen just fine too, but yes you wont get bezels, but id suggest living with that as again, the retroarch core is terrible, ill aslso say avoid retroarch for MAME, Citra and PPSSPP too, the standalone versions are far better in those cases.