Smelting is extractive metallurgy. You just wanna melt the lead.
If your molds are mouldy, wash well with a mild bleach solution.
Choose higher temperature molds. You can get much higher temperatures with other materials like some epoxy's and resins, the latex rubber molds are generally used for plaster works.
Alloying tin with lead is not as easy as it might sound, crystals can form as the solution cools and come out of sol'n which is why most (practically all) bullet casters use 50/50 alloy from a bullet casting supply, instead of trying to mix from pure metals.
In fact, if you aren't careful when you melt your own wheel weight metal, you may find the antimony, tin, and lead separating out unpredictably and giving you problems. Bullet casters know all about antimony clog of the nozzles, and the tin pooling on top of the melt and adsorbing onto the dross through capillary action, it gets more complex than you'd think.
Metallurgy is no joke, follow instructions, be patient and prepared for the occasional suprise. Alloying metals isn't just heating them both till they melt and mixing together, you often need special procedures, fluxes, magic helper elves...etc..etc..