I continue to be impressed by the quality of feedback and help on this forum!
All of the suggestions made so far are very useful and ultimately, the solutions will be a combination of practicality and what is available/affordable by way of materials.
Re skids, Jenkinsph pictures are excellent and 3' long seems about right, so does setting them just an inch or so below the teeth. I'll have to work out the best and strongest way of mounting them to do this and a "wrap around" plate welded to the existing side-plates with some internal reinforcing would probably do the trick. Making the skid height adjustable might also be feasible because adjusting the height of the teeth is not as simple as drilling extra holes in each one because each has a "step" cut into it below the angle iron mounting plate to prevent it from lifting. I think it will be better to work with the device for a while, then cut them down is it proves necessary
I think the skids should NOT be extended forward beyond the alignment of the teeth because that would inhibit pushing branches etc longer than the blade
Adding "brush teeth" to the rear blade like the Chinese ones doesn't look too hard to do. They would certainly help the cleaning up process without scraping earth so I'll probably make this addition. The idea of side plates (wings) is a great idea thanks. I do not need rippers on this assembly because I already have a separate single tine ripper for heavy work (though that means changing the entire rear assembly).
Potential damage is likely to occur only to the teeth themselves (the reason they are removeable). The main blade has been bashed about many times during its lifetime and apart from scars, shows no sign of real damage
Thanks again guys - will keep you posted on developments (might be a while though)
Alan