About the Infinite guy in the Sonic Forces trailer, I kinda have a theory, though it might most probably be wrong... But I'm gonna write it here.
I got the feeling that he might have something to do with the custom character.
Why? I'll explain.
Look at the poster. The custom guy in it is blue-ish, and the new villain is blue-ish.
Look at the trailer. The custom guy is red, ans the villain is red.
Not mentioning they both had big pointy ears on the sodes of their heads.
But anyway, it's just a theory. (insert Game theorist joke here)