This is a really helpful guide. I did the mistake of joining the Rosaria's fingers covenant before finishing Sirris quest, but luckly I didn't equip the covenant item and never gave any Pale tongue to Rosaria, so I was still able to help her defeat Creighton. Also, you really have to move through the force field and light the bonfire for her sign to show up on the bridge.

Some of the quests still have mysterious triggers though, like the one with Greirat. I've been reading about people following all the right steps and still finding him dead in the sewers. I'm still not sure how talking to Onionbro at the kitchen affects Greirat fate, but one of the theories is that lighting the Irithyll Dungeon bonfire moves Onionbro to the dungeon, so he won't be there to save Greirat at the sewers. I don't know if sending Patches to save him changes that though, but the best bet is to stay away from the dungeon before saving him.

What I did to keep Greirat alive was find Patches in the Firelink Shrine, get trapped by him as usual, buy his Catarina armor, throw it down the well to Siegward, cross the force wall at Irithyll bridge, return to Firelink and send Greirat to search for stuff and instead of talking to Onionbro at the kitchen or talking with Patches, I went straight to the Smouldering Lake and killed the Old Demon King. Seeing as killing bosses makes the story progress for most NPCs, when I returned to Firelink Shrine, Greirat was there thanking a mysterious onion knight for rescuing him.

On a side note, does anyone have a sure way to trigger the cutscene in the Cathedral where Patches tricks you by lowering the bridge? I never managed to get him to spawn there.