I really like the 2226 and 2123 together. You can crossover at decade points, 200Hz and 2kHz. I haven't tried the Vifa DX25 with that combination, but I think it would work very well. You could even run them all with extremely simple crossovers, just a coil on the woofer, a coil and a couple of padding resistors on the mid and a cap and damper resistor on the tweeter. Should be silky smooooth.

As for substituting the 2123 for an Alpha 10, it cannot be substituted when used in a bass-reflex cabinet, but it will work in the π midhorn. I can't say exactly what the midrange circuit should look like when the 2123 is used in a midhorn on a seven π, because I only tested the 2012 of the JBL line.

The midhorn in the seven π is a bit more sensitive to crossover components than some of my other designs, because it depends on the behavior of the voice coil cover for the top end range, and there's a bit of balancing between circuit inductance, cone geometry and horn shape. So this is one I can't really model well because, in this case, rising response doesn't necessarily tell where sound is radiated from. If high frequencies are coming from the dust cap, the horn's high frequency output will increase but if not, they won't. And that is largely what determines the value of midrange circuit inductance.