my guess is Merriweather will redshirt now that we're this far into the season. Speed isn't really the issue for Koerner. If he just catches the easy INT on the first play of the second half game is probably different. Minnesota is really, really difficult to play against, they put your safeties in very hard situations with the RPO. Koerner got caught flat-footed on the TD pass over top of him, that's not a speed/athleticism thing that's just getting caught moving forward during the zone-read mesh, which is exactly what the offense is trying to do. Penn State's 4 and 5-star safeties did exactly the same thing last week.
If I were to guess, Koerner will start 2 more years at safety. That will send fans into a rage, but he's got enough speed and athleticism to be good. The year of experience he is getting will make him better next year.