The title alone should suffice to say there will be spoilers throughout.
Let's say I give the Eyes of a Firekeeper to the Firekeeper, as well as follow Yoel and Yuria's questline as necessary. Then I kill the Soul of Cinder. At this point, standing in the empty arena, I haven't interacted with the Firekeeper's summon sign or the bonfire. Can I at this point return to Firelink Shrine and do things to alter which ending I'll receive, such as killing the Firekeeper to take back the eyes, or kill Yuria, or heal the Dark Sigil? I'm specifically asking for the sake of me being able to summon NPCs for the finale, but wanting to get the trophy for the "To Link the First Flame" ending.
Best Answer
According to this forum discussion, starting on the third page, it is possible to kill the Soul of Cinder and back away from the flame to do things to alter the ending that can be received. On the next page of the thread, it is confirmed that the Dark Sigil must be healed, at about the cost of the number of Dark Sigils held multiplied by the cost of the next level. Sadly, one can't simply kill Yuria and get the To Link the Fire ending.
EDIT: I confirmed myself that this does in fact work.