So are you trying to say that Elaine "Who we already know as the MC's dad's girlfriend" is his dad's secret girlfriend? What?
no... Elaine has the same motives as the phantom third heir for wanting Mc out of the inheritance.
Jolina says it was a brunette woman, seeing the photos she discards Monica and Zarah (who is blonde at that moment, but I don't think it's important), while she's not sure seeing Elaine's photo. the latter then repeatedly avoids going to Jolina's Café with MC
now there are no other brunette women in the game, so there aren't many hypothesis left: either Elaine is the one who sent the email, or the fact that Elaine doesn't want to go to Jolina is linked to something else (maybe Bella and Jolina's relationship) and there's a new character who has yet to be introduced., or Jolina is lying and wants to frame someone
The best evidence in Elaine's defense is that it would be trivial, but the clues so far say that.