Looks great! Not that i really know anything about plumbing...
I like the Legos analogy as well. I hope all goes well with your water test.
I'm glad I made it so the two circulation loops could be isolated. I started to fill it all up and found a pretty bad leak coming from around my MD7. The housing has a big crack in it. I'll have to fix that later, but...
Since I urgently need to get it cycling, I just closed the valve on that side of the filtration loop, and filled up the other side. Everything is holding pretty well at this point.
I'm filling it up about 25% a day, to let the stand, tank, and everything get used to the weight. Afterward, I'll drain it all out and put substrate in.