Good god, LokiD, why would Marvel try to create a byzantine plot like that which is (a) too complicated and (b) so far away from canonical Jan --- either 616 or Ultimate ---
and then try to attach it to a cameo
in a movie that is titled "IRON MAN 3.....?"
Stop trying to overthink these things. People go to Iron Man movies to see Iron Man.
wants to go to an Iron Man sequel and watch some long, drawn-out subplot that involves some chick that the general audience doesn't even know trying to track down clues to the disappearance of her ex who isn't even slated to appear in the MCU until 2015.....???
Again: this is "IRON MAN 3," *not* "Ant-Man: Prologue."
If....IF....by some bizarre chance, Szostak does indeed wind up playing Janet Van Dyne, I can almost guarantee you that she'll have no more than 3-5 minutes of screen time, and there'll be nary a hint that she will become The Winsome Wasp at some point in the MCU future, nor even that she's attached to the Ant-Man story at all.
This movie is about Iron Man getting "back to his cave," taking on the terrorist leader known as Mandarin, having the Extremis experiment go horribly wrong, sort out his relationship with Pepper Potts, and deal with his frenemy Col. James Rhodes, who is himself juggling an identity crisis involving War Machine and Iron Patriot.
There is absolutely no place left inside an already jam-packed subplot buffet to try to introduce the Hank and Janet Show in such a bizarre and twisted and bloated manner that would not only confuse the hell out of general audiences, but dedicated Avenger fanboys like myself as well.