I imagine the reasoning for Evan Peters casting is pretty straightforward.they need actor to play as Fake Pietro. It has to be recognizably fake so Aaron Taylor Johnson is out. But it's good if they can get someone that is recognizable as Pietro/quicksilver. Any random actor and the audience reaction would be who the hell is this guy. But with Evan Peters, even without any dialog or show of power, the fandom already recognize him as quicksilver.
That's not true at all.
Random person shows up at the door.
Wanda: "...Pietro?"
Darcy: "She recast Pietro?"
There. It's almost what already happened.
But casting Evan Peters gets 1000 youtube videos speculating on the multiverse only to end up being nothing and now we're just the idiots.