Common law marriages among non-Catholics, I sometimes view in higher regard than ones registered with the state. No clergyman was mandated until later in Church history.
Clergymen were mandated by the Sacred Canons since the Council of Trent, meaning that any clandestine marriage between a member of the Church and another or any clandestine marriage between two members of the Church is invalid. Such a marriage is only validly the sacrament of matrimony if there is a real moral impossibility of having an ecclesiastical witness. Even if said ecclesiastical witnesses may be hard to come by today, it seems rash to suggest that anybody should procure a marriage outside of the received regular means. There are just reasons for which the canon regulating this was originally passed, and it leaves very little room for exceptions. Better to not leave it to chance and the individual opinion of laymen.
This might be a question of accuracy, but when the Sacred Canons of the Council of Trent are invoked, and more so when speaking about the grave matter of questions of validity, the Church's promulgations should be followed ad litteram.
The Sacrosanct Council declares, not in a Canon but in a declaration on the reform of matrimony following the Canons, that the Sacrament of matrimony is only validly confected if celebrated in front of the proper Pastor (or another priest licensed by the Pastor or Ordinary).
No mention is made of clergymen or ecclesiastical witnesses not endowed with the necessary judicial prerogatives. I am sure you are aware that the Council Fathers and learned divines would not use this specific language in vain.
But maybe this is exactly what you mean in saying that these ecclesiastical witnesses may be hard to come by today. Or maybe you are of the opinion that the Sacrament is validly confected only in presence of priests "with" supplied jurisdiction, even though this would seem to contradict the literal sense as well as purpose of the declaration of Trent.
Or the present extrajudicial priests serve no purpose in the confection of the Sacrament and are merely present for reasons of wisdom, splendor & decorum etc. Nevertheless, one would "leave it to chance" and follow a private opinion if not celebrating the Sacrament in front of a Pastor.
Whatever one may say and whatever position one takes in regards to the crisis of the Church, one should not forget about the realities and judicial structure of the Church or even supply them by one's own accord (as sedevacantist clerics have done in assuming ordinary jurisdiction).