Diffraction and its effects . .

[YNWaN]

From the drive there is frame resonance (principally from the HF drive), and from the cabinet there is case resonance. The diagram implies there is a short wavelength waveform travelling almost like a skin effect from the tweeter. Is this intended to indicate the drive frame resonance?

No, the diagram represents the actual sonic output of the drive unit and the secondary diffraction of this sound wave as it meets edges such as the edge of the cabinet, or tweeter mounting plate etc.

With tweeters face mounted on the baffle, could the drive resonance not be better deflected by a stubby cone fitted around the drive or rebating the drives?

Both of these options will effectively horn load the tweeter to some extent and significantly alter its output. Even then, secondary diffraction effects will still exist at the edge of the cone, or the hole that the drive unit is rebated into.

As my own DIY's were deliberately built with extra thick mdf and chamfered on all edges to reduce and mitigate cabinet edge resonance, and the flush faced drives filleted in rebates to deflect drive frame resonance, I thought I had dealt with all the possibilities.

Both of these are excellent techniques for dealing with this problem. However, a sharp edge of a 45 degree chamfer will still cause an element (but reduced) of diffraction. Many speakers have rebated tweeters but relatively few use chamfered cabinet edges.Even then, in many cases, there exists the potential for diffraction to take place where the actual tweeters are mounted to the tweeter face-plate (although Jim's solution doesn't address this issue (some tweeter manufacturers seem to consider it more than others).