Give twelve grains of Blue Dot a spin. Ten of Unique. If Unique does well give Universal Clays a spin. Same burning rate but much better flowing than the flake. Your case has less capacity than the 45-70 (I figure you know this already) and staying to the low end of pistol charges for it should be alright.
You can crosscheck data with 410 loads, seeking to balance powder charge weight with payload weight, but I'd avoid H110/296, they don't do well in low density loading. 4227 is likely a good alternative.
That the 4198 is very dirty says the powder is not getting up to a good working pressure, and like you say, adding till it does could be the less prudent approach Faster powders are going to burn cleaner than slower one's will.
Have you considered a 452-454 300-350 grain bullet?
Interesting project, keep us posted. There has been lot's of interest in making the 45/410 work as a light 45 caliber rifle, you're the first to actually post that they have fired something. Keep up the good work. What twist rate is that barrel???