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Well, of course they are not invalidating. The Church teaching on matrimony is that, if two single people of canonical age who have no other impediments to marriage exchange contract matrimony, their marriage is valid, and lasts until one of them dies. In the example you give, the immaturity of the people you describe does not prevent them from contracting marriage, and people like that married validly before Vatican 2 and do so to this day. Whether they intend to keep their promises or not, they contract the obligation, along with all the sacramental graces necessary to live in that holy state.
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The entire notion that someone who is, to use your words, a "hot mess" is unable to validly contract marriage is completely false. Obviously, if it were true, it would be possible to tell people they can't get married  beforehand, just as the modernist church claims to go back in time and evaluate people's mental state on their wedding day when they come in to get an "annulment", but obviously that doesn't happen. Short of someone being mentally retarded or insane, mental problems do not invalidate matrimonial consent.

Well, the Newchurch treats this kind of immaturity and lack of discretion as invalidating factors, at least in actual practice when it comes to reviewing petitions for declarations of nullity (which are granted far more often than they are not).  But if they're going to "be this way", you'd think that the very least Newchurch could do, is to be more demanding than in times past, to "catch" these supposedly invalidating factors before the wedding, and thus keep these people out of the tribunal in the first place.  As I always say, "quality on the front end", or (referring to the factors, not the spouses) "garbage in, garbage out". 

People take such great pride nowadays in these huge, lengthy "pre-engagement" and engagement periods, often spanning years, until Bob can finish law school or medical school, or until Julie can get her career going and possibly make partner in her firm, and so on --- of course, they're almost always living together and having sex, so there's no real urgency about getting married --- with the mentality that marriage is a kind of "capstone" achievement, the denouement to a success-oriented trajectory in life.  If they are willing to wait that long for secular reasons, why can't they wait maybe a year or two, go through an extensive vetting process from a spiritual and psychological perspective, possibly making retreats and seeking intensive spiritual direction, to make sure that there are absolutely no impediments that could ever be used in an annulment tribunal? 

Anything less seems like providing a built-in "bolthole" for annulment in case the marriage goes sour somewhere down the road.