It sounds as if there’s a lack of understanding here about what constitutes a valid vs. invalid marriage, and WHY one would determine that the marriage was not valid. First, let’s talk about what constitutes a valid and invalid marriage.
A valid marriage is one that was contracted free from impediments, AND (if one or both parties was Catholic at the time of the marriage) according to canon law. IF it can be proven on paper that the marriage is not valid (e.g. things like marriage outside the Church without dispensation, prior bond of marriage, being too closely related) the marriage is de facto invalid. Now, if it’s something relatively simple, like just not having had a Catholic wedding or not getting dispensation, a simple convalidation (or radical sanation) will make the marriage valid. Problem solved. If there’s a problem such as a prior bond of marriage, a declaration of nullity for any previous marriage(s) can be sought, and if and when that declaration is given, a simple convalidation or a radical sanation will make the marriage valid. If there’s a permanent impediment you can prove on paper (e.g. pre-existing, permanent impotence or being too closely related) a valid marriage is NEVER possible between these two people.
If it CANNOT be proven on paper that the marriage is invalid, it’s considered valid until proven otherwise. Marriage always enjoys the “favour of the law” meaning it’s considered valid until proven otherwise (similar to the “innocent until proven guilty” in a court of law). This means that factors that are more nebulous, such as coercion, intent not to have children, lack of understanding of the nature of marriage, etc. (things for which you need witnesses and evidence) require testimony from the petitioner, witnesses, and possibly the respondent (if the respondent chooses to participate) in order for the marriage to be declared invalid. In our area (Alberta) one is required to be legally divorced prior to seeking a declaration of nullity. (You have to provide a divorce certificate as part of the required paperwork.) Consequently, if there’s no tangible reason (i.e. one that can be proven on paper) for a marriage to be invalid, it’s valid until proven otherwise. Happily married couples without any visible impediments may consider themselves validly married.