Regarding the charcoal fines: If you just add the fines to the charcoal supply, I would expect them to be carried away with the gas flow in the filter or cause clogging already in the hopper.
One possibility would be to add it to the air stream into the nozzle. Here, the fines are blown directly into the hot reaction zone and are soon converted into gas (principle of the Källe-gasifier).
Too much at one time could cause clogging, too. You would need to try out a feeding device.
In the past, some gasifiers were designed to use the charcoal fines or to also at least work satisfactory with them (Källe-gasifier or Mako S-gasifier). But this would mean you have to build a new setup, which I think is not your intention.
Pine cones: There is no reason not to do it, but maybe the char is brittle or has a high ash content and is thus not ideal for a gasifier.
Hose diameter: In general: Larger diameters and “inner streamlining” are beneficial.
Your hose does not seems too restrictive. But if you want to know it exactly, try to measure the locations of the pressure loss over your system (gasifier, filter, hose from filter outlet to engine).
Air mixing valve: Well, I personally would prefer to have a valve to shut down the gas supply to the engine completely, just in case there is something wrong with the gasifier.
Timing advance: Your engine will run without timing advance or just a little bit earlier, as you have already proposed. Also, like Thierry wrote, hydrogen rich gas is easier ignited and burns faster, thus needs less advance.
However, the sweet spot may also move due to changing gas quality.
Long story short: If you just want to cruise around, a permanently setting will do. If you look for the least power loss on chargas, make it adjustable from the driver’s seat and try the best settings during the drive.
Btw: You use water drip AND exhaust gas return (EGR), right? Best is to use water OR EGR, because both is heat consuming in the reaction zone and thus “stealing” the needed heat from each other and could so lead to lower quality gas.
Hope you find it useful, regards,