MEK will just evaporate as you figured, look at how fast the liquid “Testor"s” will evaporate compared to other styrene cements. I don’t know the actual formula or added products in the Model Masters or Faller, but from noticing the way they perform, I am assuming that there is a additive to slightly thicken and slow the drying. The reason I really like the Faller Xpert is that the cement will gail an initial hold yet still allow for some minor repositioning of the parts. You can also wipe off any excess w/o marring the surrounding factory painting surfaces ( of coarse done rather quickly) This will not happen w/ straight MEK.
These additives to other cements are the dried parts that are causing the clogging in the first place.
Amazing how much we can talk about glueing to pieces of plastic together…