this is an idea:
if you are authoring those models, you can, when authoring them, position the uvs with a padded size.
so that when merged, they don’t overlap. i’d need to draw it, but i think it’s clear enough.
that probably means that your textures are going to be larger. as they all will be padded.
i don’t think you can build lightmaps with overlapping uvs. i think that’s just a limitation of the technique (not even the implementation).
afaik a lightmap is a texture, so it’s ONE texture per MESH.
since you want to have ONE MESH then your uvs can’t overlap.