You answered it yourself. Edman backs up DeJong at SS and Schrock backs up Edman at 2B. Its not like Shildt is going to rest two starters the same day. This would be very workable, IMO. The bonus is Schrock’s LH bat off the bench, which right now, might be the only one with Wieters and Brad Miller gone.
P.S. Folks here seem to not be aware that Schrock was primarily the THIRD baseman for Memphis in 2019. So he could either back up Carpenter or they could slide Edman to third on days Carpenter sits and play Schrock at second.
Bottom line, Schrock may only be an emergency SS, but the Cardinals are not really disadvantaged if they choose him over Sosa, who can play all three positions.