That is plan B, add lead to unknown-alloy scrap. Yes it seems the pure metals are more easily sourced (compared to specific brass alloys).
Yeah, I know it's an old post, but the info is good, at least in my opinion.
I may be biased a bit...
Skip the unknown alloys.
Scrounge copper (plumbers, electricians, probably wise to pay them what they would get as scrap), zinc is easy, used in a lot of tire wheel weights [marked as Zn.] these days, as well as for sacrificial anodes for boat motors and hot water tanks, and lots of other places). The bullet casters that used to be hot on scrounging wheel weights at tire shops have cut way back, due to the reduction in use of lead in wheel weights. If you know a guy that casts his own bullets, he may have a bucket of zinc that he will hand along! You only need 3/4 pound, for a 15 pound batch.
Lead Free plumbing Solder is bout 97% tin with traces of copper and silver making up the rest, in most of it. Tin ingots are not that hard to source, but for the 'easy button' a roll of Lead-Free goes a long ways. Pricey though.
I have read of, but do not have offhand, the composition of, some shot that the foundries used to use, when pouring brass, to 'innoculate' the flask, just before the pour, essentially to replace the elements lost during the melt and subsequent additional heat in the furnace. May be worth looking in to, may not be... Have also read of using glass or similar on top of the melt, to keep it from absorbing gasses at temperature. More reading...
You may know, but others may not, that welding or melting and overheating Zinc can cause a lot of white smoke, which if breathed in, can cause severe headaches among other side effects. Puking too, if you get enough of it. Not fatal, but miserable. Drink milk, if you are welding Galv, or you cloud yourself out, in the foundry. Big white snotty clouds of the stuff in the air, the first time I oveheated a brass melt!