Just my opinion, and I fer sure ain't "medical" or "professional", but... It probably takes as long to get a "true" mctd diagnosis as it does one for sle, since you're talking about eliminating all sorts of almost similar ai diseases. MCTD is like a combination of RA, SLE, Raynaud's & Sjogrens, etc., but not the full-blown version of most of them, and it seems that there are as many opinions of what exactly mctd is, as there are rheumatologists' groups. It's a relatively new name for ai disease. The kidneys are usually not involved. Other than that, it looks like a duck, quacks like a duck, etc..., but doesn't have enough evidence to call it lupus. I was told I had that by two different docs over the years, and told by a couple others that "that's ridiculous". ymmv. As to whether it's a catch-all for "no clue", I'd lean more to "interim or intermediate diagnosis that allows for identifying and treating a given disease condition." ahem... but they do want to treat with meds in an attempt to lessen its impact on you. I'm familiar with plaq, but not the other two.
"There but for the grace of God, go I."
"... His mercy endureth for ever."