So I got in there this am and put the wheels back on the front, then lifted up the rear a few inches to put the end of the main brake line above the height of the fluid reservoir. Put garbage bags on my seats in case I'd have to pull out lines in there between them, and then managed to extract the troublesome manifold and its attendant brake lines.
I could see that the T junction was not split again, so in reconsidering those steel braided lines—which came in bags with the rest of the parts I bought with the kit, I compared the male end of one with the bubble flared fitting that's supposed to go in there. Interesting:
It was these fittings which had given trouble before. So much so I gave them each a wrap of teflon tape before screwing them in last time.
In consultation with Cory Drake I learned that fitment issues with these are very common, if not universal. He suggested I replace them with "stock" hoses and, digging through my box of brake lines I did come up with something a bit less flashy:
About three inches shorter than the steel braided lines, but by then I had already decided to put a short length of hard line into the T junction to give it something it likes, and I had a spare male/male barrel fitting in stock.
Comparing the business end of the "rubber" line to the braided line we see maybe what the problem has been:
I'm thinking that the beveled edge, in steel, is different enough from what is supposed to fit in there as to account for my troubles.* Fingers crossed on that.
So I reassembled the whole mess on the bench and then popped it in the car.
Went out to buy a power bleeder but it's Easter, ya know, so I just pumped the brake pedal a little with a mop while I opened and closed the bleeders. Seems like maybe it will work. I'll get an assistant back in here tomorrow or Tuesday and—I hope—finally get the brakes squared away.
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*Hat tip to Gordon as well on this.