Ive been doing the MCC master chief saga playlist to get the life story achievement. I came to the end of the halo 3 level the covenant and realized that if 343 GS had an index or could get one to activate the ring they could have just went the faster and more easier route rather than have the chief delve into pretty much the graveminds house to get cortana. Is it ever explained in the Forerunner trilogy how an index is made and if a ring was ever to be replaced would a new index be created?

Whatever the reason being to activate the ring faster would john do it and leave cortana behind or would he risk himself to save her no matter what?