You're right that flare angle sets length if throat and mouth area are held constant. What it's going to do is to give you a truncated horn. You'll probably find that it will work well in corners, but the low end response will sag when pulled away from the corner. The corner actually becomes the horn for the bottom octave because the flare alone is too small to support the lowest frequencies. So if you pull the horn out further than about a foot away from the corners, you lose all the bottom end.

You may also have some issues with your upper crossover point, and might have to shift it down just a smidge, both because of midhorn rolloff and because of summing. That's where your real work will begin. I'm hoping if you don't modify the horn too much from what I already have that the crossover will work well as it is, or with only a slight shift. I'm very proud with how seamless the integration is.