Welcome to the forum.
I would copy your question and start your own thread on the topic to get more participation and answers.
The EPA is causing all lead wheelwights to be phased out nationwide. I think it is by some time in 2011; not sure about that year, but it is coming soon. So WW alloy will cease to be available in wheel weights.
Zinc is the trouble maker as it interferes with mold fill. As long as you keep your melt a good 50 degrees or more too cool to melt zinc (787.1°F (419.5°C)) the zinc weights should float to the surface, too.
Here is a good discussion of alloys and I think it will answer your questions.