A priest's opinion doesn't give you certainty as you suggest, only the Church can give you certainty with a declaration of nullity, which is at present, unfortunately, unavailable. The SSPV is so indifferentist that it doesn't even require general confessions for people converting from the Novus Ordo. It's useless to gamble on the opinion of some priest and then if one doesn't like that priest's opinion he can go in search of another fallible opinion.

Catholic principles in this case are clear-cut and anyone can apply them for himself - unbaptized non-Catholics marry validly when they seek a civil marriage, that's certain, the question is only if that's what happened and StrivingCatholic knows that for himself. In any case, to go seek another marriage a declaration of nullity would be required, which he cannot get, since there is no competent authority to provide it at present.

In short, one can and should seek advice from knowledgeable priests or laymen, however, one can't go seek certainty from a priest's opinion.

P.S. The marriage is assumed valid until proven otherwise beyond any doubt, not the other way around.

No, but thank you for asking.  I know that is a common reason for people leaving the Church.  Also, thank you to everyone else.  I picked up a copy of the Fortescue book as well as one by Erick Ybarra (a novus ordite but he is a scholar on the Greek Schism) called "The Papacy."  I already pray the 15 decades but of course will continue to do so.  Prayer and study. 

Yes, i'm not suggesting a layman can make any decisions related to canon law.  I'm suggesting (and i've known examples) where Trad priests investigated the matter and the person converted to the Faith and was "re-married" to a Trad.  But the priest made the call.