You cannot convalidate a marriage with an impediment that is not removed or dispensed or fixed somehow. If one (or both) parties remain incapable of real consent, for example, how could you convalidate? If you did, it would fix nothing, no matter how willing the parties.
Right. That’s why people seek Declarations of Nullity for previous marriages. Things like immaturity can and often do resolve themselves over time - an impetuous love affair can mature into real mutual self-giving, for example. Insanity and addictions are medically treatable. Illness isn’t an impediment to marriage; neither is sterility. If the priest let you get married in Church, there was nothing obviously wrong at the time - which means you have to decide what you can live with - and if you decide you can live with it, then your marriage is valid.
Why would the powers that be ask them to divorce? I don’t know. Because they have been classified as not married and won’t be for the foreseeable future (and there are no children)? I only imagine that could happen if someone official classified them as not married.
They will never classify a couple that is married under the civll law as “not married.” That simply won’t happen in the Church. But if there is lack of form, they will certainly require you to have a convalidation of the marriage, in order to dot all of the "i"s and cross all of the "t"s.
But even with clear lack of form, they will still consider you “married” until a Declaration of Nullity is decreed, which would only happen after a divorce.
If your spouse’s insanity (for example) was “cute” at the time of the wedding, but is now making the common life impossible, you have the right under canon law to separate and even to get a civil divorce, but you must continue to consider yourself validly married to that person - the separation or civil divorce is for safety reasons, only - it is not a prelude to remarriage.
You also have the right, after a civil divorce (which presumes that you have no plans to move back in with the insane spouse at any time in the future) to have an investigation done by the Tribunal to see whether any other impediment to a valid marriage existed, but if you were aware of the insanity at the time of the wedding, and you accepted it as part of the person whom you love forever, then you are validly married. It’s only if the insanity was kept a secret from you that it would be an impediment to the marriage - and only if you promptly divorced the person at the moment that you found out about it. If you lived with it for ten years, then it’s part of the marriage, and your marriage is valid.