Isn't a mixed marriage only considered "valid" as a "natural law" marriage? It surely isn't a sacramental marriage, so it's not valid, from a sacramental/Church aspect, but only valid from a human promise aspect. This would all be true even if the couple was married by a priest, no? He can bless the marriage, but it's still not sacramental. If the couple didn't get Church approval, it would be a sin. If the priest performed the ceremony without approval, it would also be a sin.
Certainly if any catholic gets "married" by a non-Catholic (in any shape), that is not a catholic/sacramental marriage. In fact, such an act would be a grave sin. It would not be a marriage at all, neither natural law or sacramental. The best the couple could hope for, spiritually speaking, would be if they stayed together and God blessed them as fulfilling their natural vows. But spiritually speaking, this would be the worst-case scenario for a catholic, only slightly better than co-habitating (but maybe, from a moral theology standpoint, the same grave sin as co-habitating).
I don't mean to question your traditionalist bona fides
, but some of this just doesn't sound right --- "valid from a human promise aspect", "God blessed them as fulfilling their natural vows" --- God does not bless people for sin
. It's not "slightly better than cohabitating", it is
Here is how I have always understood it:
* Catholic marries Catholic with canonical form - valid sacramental marriage
* Catholic marries Catholic outside of canonical form (outside of the emergency situation where a priest cannot be had for a month) - invalid
* Catholic marries non-Catholic Christian with canonical form - valid sacramental marriage
* Catholic marries non-Catholic Christian outside of canonical form - invalid
* Catholic marries non-Christian with canonical form (i.e., dispensation) - natural marriage but not sacramental
* Catholic marries non-Christian outside of canonical form - invalid
* Two non-Catholic Christians marry - valid sacramental marriage
* Non-Catholic Christian marries non-Christian - natural marriage but not sacramental
* Two non-Christians marry - natural marriage but not sacramental (but if one spouse becomes a Catholic and the remaining non-Christian spouse will not live peaceably with them, the Catholic may remarry per the Pauline Privilege)
If any of this is incorrect, please show me how.