Well, time for a photo update.
First, I inventoried my remaining plumbing parts in preparation for my This Will Be My Last Order order...
...which I placed with a combination of Savko, AquaticEco, and Marine Depot tonight. I really hope I am done with the majority of the expensive plumbing pieces. I also bought several extra of all the cheap pieces and one each of the expensive pieces in case one blows up, breaks, or I realize I forgot something. I definitely have enough plumbing en route now to cover me for a while. I hope.
Here you can see the sump and the plumbing that I have been battling with for the past week. Doesn't look like much, huh? I now have drips on two of the threaded joints again, so I'm going to have to pull them off again and retape. <sigh>
The right-hand valve going to no where is where the second Dart will be connected. Dart 2 will provide flow to the refugium and prop tanks, and any extra will be diverted into Bertha. Sort of a backup pump and distribution pump in one. The middle 2" bulkhead is reserved in case I need an extra pump, but I'm thinking I will be fine... and I'd like to avoid taxing the sump with more flow anyway. I'm also quite worried that the sump may be a little too small... I have about 10 vertical inches of play left in the sump when it is running at minimum safe water height, and each inch is a little over 5 gallons. When the tank as plumbed right now drains down during an outage, I use up 4 of those precious inches. The other six will have to be enough to account for all the draindown from the much longer returns from the other tanks plus whatever the skimmer will dump out. It is going to be very close.
Here's an angle that shows the drains coming out of Bertha. [It's a little messy at the moment... try to disregard the cords, vinyl tubing, and crap everywhere.

] There is only one active one right now, and it seems quite capable of handling the 2000gph that the Dart is able to push through that flex sweep and three sharp right angles. The flow curve says 2400gph at 6 feet of head pressure, so this sounds about right to me. (It's also plenty of turnover for my purposes.)
The closer, again valve-to-no where is where I hope to install the gravity-fed drain to the skimmer. The theory will be to feed all of the return flow through the skimmer and, via a tee, down the sump via that closest drain. Then, the drain in the back will become an emergency drain. There is a third drain port in the middle there that is just a reserve in case it's needed one day. (Unlikely.)