you are not the first person reporting issues in assura LVS with umc18 PDK, I do not have Assura so my help is limited in that area. If you have Calibre it works well in that PDK
As of the contact problem, it is not clear: which contacts are you missing? you keep saying that you were able to use m1-pdiff; the contacts are defined in the tech library and there are no context files to load explicitly... did you attach the tech library to your design library? please provide more details if you want some help