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2- For semi-rigid diaphragms, the contained areas are recommended to be assigned with the same structural properties of the base slab in lieu of null areas. The latter may cause local modes when their load/additional mass are part of the lateral mass source.
3- Refer to the release note below for the enhancement that was recently implemented in ETABS v19.1.0. This new behavior affects ETABS v19.1.0 and later versions, this is in line with the current behavior of SAFE:
"An enhancement was made to make uniform area loads assigned to floor objects with properties to be additive if the objects overlap at any location. This was already true for floor objects with null properties that overlapped objects with properties. Older models where drop panels overlap slab areas and have identical uniform load specifications will have their duplicated loads deleted to maintain the same total loads"