Definately get a gas check design, as accuracy will tend to be better than plainbase, and pressures lower with heavier loads.
So far as addition of tin to ww alloy. I recommend only a short piece of no lead silver bearing plumbers solder to a fresh pot of alloy.
This solder is available at all hardwares which sell plumbing supplies, but read the label and be sure it contains some silver, as a trace does magic to castibility, for only pennies per potfull. Standard diameter is 1/8 inch, and 2 inches for a 10 pound pot or 4 inches for a twenty pound pot will improve castibility and put a little shine on your bullets. You'll not be able to find any other benifits that I can detect. But it is so inexpensive for benifit it gives. By the way, it will really show when casting pure lead muzzle loader bullets on much of the lead sold as pure or soft lead, and not change or up hardness at all.
Purchase rifle throat slugs from LBT for rifles, and push through slugs for both rifles and handguns. Don't use hard cast bullets for slugging. Only very soft, or nearly pure lead. If you have a mold that casts large enough, get it hot, cast with WW alloy, air cool them till just cool enough to handle, and oil them a bit and they will slug to the exact diameter of the hole they pass through, which should be only cylinder throats. You probably won't be able to get them through a barrel, and stuck bullets are no fun, even for a professional smith to remove.
My experiance with Lil'gun is zero, as the Feds stopped me from shooting 7 years ago. (But they haven't stopped my business -- YET! Which is their goal.) Look in a burning rate chart. If it burns at about the same rate as H110 / 296, it should be great. AA 9 burns about 10% faster. But the most attractive aspects of these powders is a straight pressure climb to 100,000 psi, where most if not all flake powders take a spike at about 40,000 psi, which is very undesirable with heavy loads which will be used in hot weather at times. Also these are ball powders which cause minimal bore erosion, far less than stick and flake powders. - I have no idea where Lil'gun stands in these respects.