Difficult to diagnose over t"internet without being able to see the installation firsthand.
I recently rebuilt some DCOEs that had similar issues to what you describe. Absolutely everything stripped out and removed for cleaning and inspection including accelerator pump bypass and choke jets. The owner shat himself when he saw 4 carbs completely stripped to bits on the bench!
These were 4 identical carb bodies and I selected the best components to make the optimum pair.
Mixture screws not having a noticeable affect to running sounds like it could be a balance problem between the carbs so the engine is mainly running on one carb which is flowing more air, or simply partially blocked idle jets which doesn't take much seeing as the fuel holes in them are around 0.5mm diameter.
Some engines are very fussy about airflow and idle mixtures / jet selection so setting them up takes a bit of finesse and skill.
Are your carbs the early type with the curled flat spring key orientating the auxiliary venturi, or the later type with the grub screw and locknut to clamp the aux vent in the carb body?
Early type can allow the aux vent to rattle around and wear a bit depending on how the trumpets have been fitted.
Fuel mixture doesn't issue through the aux vents at idle so shouldn't be the cause of fuel leaking. However, the later carbs can also have idle air bypass adjustment which is preset at the factory when new. If there is wear or a slight twist on the throttle shaft or someone has played around with the air bypass screws that could account for poor unresponsive running at idle / progression phase.
The accelerator pump jets should have tiny alloy crush washers where they seat in the carb body and are clamped in place by large brass plugs with O rings to seal them. Modern fuel with ethanol content does seem to trash O rings and cause them to leak. Fuel may dribble passed the pump jets if the alloy crush washer is missing or damaged.
Disappointingly not yet a Jackass