They use Carl when they're in a temporary bind, but methinks they'll want a woman long-term. Sharon has experience on the shift, but who else is there to work EEWC? Would they just send Rich out there alone every night? (No, don't take Sharon away!)
Betty Davis or Samantha Mohr also make sense because they're on PM Edition, which is the next logical shift to pull from due to its proximity to EE. Of course, in the past, they've used people from completely different shifts to fill in for OCMs on maternity leave. Who knows, they might even pull someone like Nicole Mitchell (YWT Travel Analyst, just like Alexandra once was) if she's up to the task of working nights instead of mornings.