I don’t know why I didn’t test it before asking 
(maybe because it’s 11pm and I’m still working on this…)
But I did test it, and threw in 3rd options (generating lightmap UVs by UE)
Seems that actually wasting UV space might not be that bad idea here as there is quite visible seam in shadow for A…
(that’s all default lightmass setting with 32 lightmap resolutiion for all of those objects)