I'm not absolutely sure of the difference either, but I'll take a stab:
The WAS has a longer build-up or lead-in time. The spouse is unhappy for an extended period of time. Eventually gives up on the complaining, which can lead to the other spouse thinking everything is fine. The WAS picks a time in the future to leave, then springs it on the other spouse.
MLC is more tied to the mid-life transition. More stable people/relationships handle this self-questioning, and keep on trucking. Others end up blowing up their marriage. I think MLC can be influenced by an extended bad R, which can make it similar to WAS. Some have said MLC is tied to depression. I think MLC would have more wild behavior associated with it. I think MLC has more of a "fog" aspect to it, and would have a longer resolution time. MLC usually has a trigger event. This is all from memory and could be wrong. I don't think WAS or MLC has much of a difference in DB techniques.
M: 41 W: 41 Married 2003 2 boys 9 & 6 Bomb Dropped May 2017