3) It is a fact that Zeref had been on Tenrou Island for quite a long time. Maybe after Mavis founded Fairy Tail, Zeira went back to Tenrou to live there, and then met Zeref, who made her into a demon.
4) Let's go back to the photo of Sayla. She's reading, right? This doesn't seem suspicious at all, correct? No. Zeira actually seems to have a hobby of reading. Here's proof.
6) Another reason is the time periods. Zeira and Mavis' time was x686. Sayla appears in x791. In that time period, Zeref could have easily made her immortal, or she could've kept dying and then being reborn in Hell's Core (Tartaros' Lab)