The big hook for this speculation revolves around a scene in the sizzle reel for The Flash that showed Iris touching the face of Savitar Barry that I don’t believe we’ve seen yet.
Really, Barry only needs to believe Savitar killed Iris for the creation of his God status to stand.
As long as Present Barry thinks his fiancé is dead, Savitar’s existence-timeline still holds true, with the real Iris perhaps trapped somewhere.
Unless that scene was cut from the final episodes, who’s to say Iris didn’t sway Savitar with romance prior to the big deadly moment so that they two could work out a plan for her survival?