I noticed some time ago folks began calling horns, waveguides, about the same time folks began to call a horn, a lens. As I was formally employed at a company that manufactured microwave antennas, the term waveguide was the technical description for what is basically a rectangle tube with an internal crossection that looks like an 'H', except with a small gap between the horizontal line. The semi-rigid cable used between microwave components and antennas could be considered waveguides as well, as the electro-magnetic energy is guided to it's destinations. In that case, the E-M energy is transmitted through the center conductor, along with what ever electrical power is required to operate microwave components.
Calling a horn a waveguide makes a bit of sense, but I think whom ever it was that began calling a horn a waveguide was attempting to describe it's function. It actually does 'guide' the pressure wave, but I'd rather call a horn, a horn, and be done with it.
I'm unclear by what you call a, "horn/CD combo". If you mean Constant Directivity, then a "CD" as you call it, is a horn.
As for using the larger horn, it was discovered after the advent of sound systems that were designed for talking movies that large horns image better, although the acousticians back then didn't call it imaging, they called it separation. Yes, stereo sound existed even back then. All that would happen if you leave the horn mounted in the enclosure as it was originally intended, rather than blocking the woofers, is that you will experience less dispersion on the horizontal plain, and more on the vertical plain. If you use, for example, a horn throat for an ALTEC Mantaray, or JBL Bi-Radial horn, in essence you would have a horn slot that would behave in the same manner as the JBL 2405 tweeter. A vertical slot by it's very nature provides better dispersion on the horizontal plane than the vertical. It's why column speakers work so well. It's kind of an acoustical phenomena.
All I can suggest, is that you try the combinations you're interested in. If one works better than the other for you, then go with what works best. It's easy to armchair quarterback, but it's your ears that will make the final decision.