Regarding the second flare angle, it's referred to by Keele in his AES paper "What's so sacred about exponential horns."
The answer is towards the end. The paper is actually about a CE horn, one that joins an expo throat to a conical with a flare out at the end, a more generous one.
The spacers aren't expected to extend the top end, just give it a minor lift.
Woofers in the horn achieve little in this design because it's large enough to extend down low. Horn loading is doing all the work. The measurements show that it's holding pattern control even at 300 Hz. What I'm thinking of doing is stopping work on S2 and moving on to the next. I'm very close to calling this one finished. Doesn't mean I'm finished building Synergy horns. This could be just the beginning.
I have ideas for another three already.