I was very interested in the Craig Gruber bit, so here's a bit more detail on all that jazz. From wikipedia.

Former Elf and Rainbow bassist Craig Gruber also rehearsed with the band, though the extent of his involvement is unclear. In a 1996 interview, Iommi stated that Gruber participated only "for a bit". Gruber has stated that his contribution was quite substantial; he says he cowrote most of Heaven and Hell's songs and that it was he and not Butler who played bass on the album.[10] Despite not being credited for his contributions, Gruber says he and the band nonetheless reached "a suitable financial arrangement".[10] Iommi conceded in his 2011 autobiography that Gruber had indeed recorded all the bass parts on Heaven and Hell, but Butler had re-recorded them upon his return, without listening to Gruber's bass tracks. Gruber later stated in an interview in 2009 that he only helped write "Die Young".