It's something to ponder that I don't know if there are a lot of answers to yet (again, glad to hear in this that a bunch of doctors are entering a new study).
I hear you on the, "Is it just another word for MLC or infatuation?", and here's where I (not being the expert) think it may be a little of both but not the whole salami on either.
When Dr. Dorothy Tennov began her studies in the 1970s and coined the term, it was more philosophical and didn't deal as much with the pathology we're dealing with, so in that way, it WAS just a study of the nature of infatuation. She still faced scrutiny for it and it wasn't until Dr. Helen Fisher took back up with a much more neuro-angle that I think we see the concept we're dealing with now. I have a bit of a soft spot for her work since I found it long before I found this site and the concept of MLC. Particularly, her writings on serotonin imbalance and dopamine suppression that turns "off" love toward a spouse. I don't have the link handy, but for the neuro-studiers, she's a good resource.
But even with Joe Beam, who DOES have an angle with this that's not just academic (he has a program for couples and LBS he wants to sell), it's consistent that limerence in general does not have a target demographic that would put it at just middle age, therefore I don't think it can just replace MLC as a term. None of these doctors go deep into the other areas of life where there will be MLC behaviors (though they do mention, Joe Beam stating the statistic of 85% of the time, that the limerent object will become a distracting obsession that can effect the rest of their lives).
But it gives us insight into the nature of the affair, and I think that has value. Yes, normal and healthy relationships can begin with a sense of "infatuation" - that's the dopamine - but most don't have a habitual aspect. There are people, or maybe something within all of us that's just waiting for the right conditions, that cause a 'tip' into something different. I'm sure we were "in love" with our spouses when we started seeing them, with the butterflies and a bit of distraction from daily life. But rarely can we compare what we experienced with them to what we saw with them and these alienators.
Think about pre-teen girls and pop idols. Two girls may both put the posters on their walls, watch the YouTube videos incessantly, go to the concerts, etc. But for one who is emotionally stable, this is part of the developmental phase of puberty and Justin fades into the background as she develops new relationships. Another may not be so stable and continues this obsession well into adulthood, set on developing an actual relationship with Justin despite the irrationality of it. I have a former friend who did this with an 80s rockstar, and it did not end well when he didn't react the way she imagined he would to her loyalty. She had a string of tragic events in her childhood around the time she developed this "crush" that I think played into developing such deep neural networks for this fantasy escape).
So with limerence, to my interpretation from these sources, it is a more pathological and unhealthy attachment that is one aspect of MLC but does not define the entirety of MLC. It doesn't require the midlife demographic to come into play, and not every relationship a sufferer has will be limerent. But that's just me!
If you want to move this to the articles, I'm cool with that.