First, this post is in serious need of an edit job. A couple of errors I can overlook, but there are a ton of errors in the author's post and, as such, it makes any argument within it appear amateurist at best.
I have no idea why the argument is based so much on professional connections that Gyllenhaal with others in the MCU. He's a fine actor regardless of those, and I think he'd do well as MK. He's not who I would select, but maybe I've stereotyped him into too much of the Donnie Darko/Nightcrawler weird actor.
I'd go with Derek Theler as Spector.