I have an old article(2002) from Guitar One that says he was GNR's guitar engineer then went on to become their guitarist. It also says that the songs from the first APC album were actually songs that Axl rejected(and Billy was getting bored) so he left with his songs to start APC.
billy didnt wrote the songs for GNR.. he was only the enginner of the record..
also the majority of the songs were written in 95-96... 2 years before he started hangin with GNR
he left gnr.. cuz he never wanted to be that long in gnr.. after all he was rite... cuz he could have stayed 6 more years waiting axl finish the record...
i think he made the rite decission.. cuz APC rules!
also he is the one who worte all the music in APC... maynard writes the lyrics and he the music...
mer de noms... was completed musically even after the band was formed