You chose the least bad option for fillers, so once the Milliput has cured, reaction will only be possible where solid filler and solid PLM plastic touch. The worst case is if you were adding lead powder in PVA glue (white woodworking glue). Over time, all the lead would be converted to lead oxide which being less dense than metallic lead would expand and burst the boiler of your locomotives in as little as a couple of years. Most large scale ship models with cast lead fittings (such as anchors) in a glass case with wooden sea base will have had the lead fittings disappear completely in about 70 years.
My Skillball's models arrived yesterday and I shall write a separate review when I have had time to prepare,build and paint some of them.