Clemens, like so many good players, played SS and CF in high school (though not college). And like Merrifield, he's primarily a 2B. Whit was already a year older than Clemens is now when he first played more than 16 games in the OF as a major leaguer. I don't imagine Clemens is any worse defensively in the OF now than Whit was then, and probably faster and more agile than Whit is now.
And Merrifield only broke into the majors himself at 27, and only had a 91 OPS+. The opportunities he got on a cheap, terrible team at ages 28 and 29 just don't exist for Clemens (or Sosa) in Philadelphia.
And right, I don't think the Phillies are trying to develop Pache at this point. Can't imagine they want to carry him all year and hope to keep him for next year, unless they demote Rojas. They're just hedging their bets until mid-season at the latest.
With the spot this team is in this season they should rule out nothing, including DFAing (or trade-dumping) Merrifield or even trading Sosa if he helps you get something else and you're comfortable with Clemens, Merrifield and Castro (since Stott can play SS too). But that means waiting to see where everyone is in two months too.