There are no formal charges against Wuerl... sadly that are public anyway.
As such pretty hard to keep him from leaving the country.
True...but if they pull the trigger on this, to most people (of which I am one) it would be tantamount to an admission of guilt, both on the point of Wuerl AND Francis.
You talk about "bad optics"....but this would be about the worst possible.
Since he has information vital to the case, they can detain him under 18 U.S. Code § 3144 - Release or detention of a material witness, and arrest anyone attempting to help him get out of the country under obstruction of justice.