I'm genuinely struggling to see what design problem this proposal is hoping to overcome. You are choosing to not use the combustion energy of the wood fuel itself in exchange for using solar, but I still don't understand from what has been written above why you think that is desirable enough to pursue? Do you find that you can't make biochar by combustion for some reason? Does normal combustion have some other problems associated with it that you need to overcome?
The external energy needed for me to make a pit full of biochar is - literally - that of one match. It burns clean and smokeless, and produces fabulous crumbly biochar. You still need to find a way to safely handle the gases produced from your solar kiln, which if they escape to the air, are the very worst set of complex hydrocarbons, smoke particles, carbon monoxide etc...
My portable "pit" is half an oil drum cut down it's length. I take it to where the brash is and just start burning. No complex set up, no need to haul bulky brash to a permanent structure.
I see other design problems as well. You mention keeping a retort at high temperature for 3 or more hours using solar. This is highly dependent on long hours of uninterrupted sunlight, and a lens setup that can accurately track the sun over that time period. Your retort will end up heating very unevenly - one side of the retort will get the direct heat from the solar reflector, while the other will be in shadow and losing heat all the time. To ensure even, complete pyrolysis you will need a mechanism to agitate the contents of the retort. I had similar problems in my old designs, where the heat would be uneven within a drum, one side would be untouched and the other near burned away to ash.
Solar for intense heating has so specific issues. Here in the cool and cloudy UK it would be a complete non-starter. In countries with enough consistent sunlight, those hot sunny days often also coincide with high fire risk days. The components of powerful lens/mirror systems can start fires at a considerable distance unintentionally, especially when conditions are dry and hot.
Overall this feels like a solution looking for a problem to solve. You could spend a lot of time, money and energy building something that works less well than an oild drum cut in half and has a whole host of extra safety and pollution issues.