I think picking up one of these names (or someone similar) and sticking with a six-man rotation (either Schwellenbach or Elder, of the current options; and then probably Smith-Shawver when he comes back from the IL) is the way to go. And I agree, of those names, Eflin is the most obvious player-to-roster and team-to-team fit.
For now, they've just got to ride Schwellenbach and whomever else they can call up to fit the schedule (with Elder looking like the obvious next-choice-up). I understand the concern with regular season W/L, but I'd rather bank on the offense rounding into form and continue to proactively manage the workload of the starting crew.