Great progress Chop. I'd love to hear these some day.
I'm glad you are continuing with the project. It wasn't long ago that you almost gave up on the project. Now you want to add cost and complexity for what would be a tiny gain. I have to admit I would be tempted to do something similar but I'm going to advise you to just get the dual 15" version done. I know it isn't as wild, but honestly you are talking about covering a very small passband with those TD18's. You already have significant headroom with the dual 15s. There becomes a point of quickly diminishing return on headroom and in my experience the point is right here.
Then you run into the issues with trying to make the drivers place nicely with each other in a pretty small range of ~500hz and below. Either you get the messiness of a .5 way where you don't really need it or you use the TD18s from about 30-200hz and td15's from 200-500hz. I just don't see much benefit.
Beyond that, you can always add later. I don't think you will care to once you hear what this can do, but I don't see any reason to slow things down now.
As cool as the Everest is, I wouldn't use it as a design benchmark. First, it has to cater to the uber highend hifi market which makes unusual (and detrimental) demands. Things like extension beyond 20khz and below 40-50hz just don't make sense from a pure performance perspective. The extension beyond 20khz is there to satisfy the people who enjoy the spec sheet more than the speaker itself. The low extension caters to the people who still refuse to use a separate subwoofer as dumb as that is. If you took away those two requirements I think they would design a very different uber-speaker. In fact, I think it would be closer to what Chop is doing now.