Question 1: So it sounds like that could be a bug you are trying to report and not as much a question. However, if a single landscape (no context as to the size and overall resolution) is 30 MB and when you create 10 of them it moves up to 300 MB, that seems expected. 30 x 10 = 300 so the “bloating” you are reporting seems to be expected. Whether they are identical or not, they will still occupy the same amount of memory as their baseline cost. If you set your texture samplers to ‘Shared:Wrap’ that can reduces the overhead by allowing your samplers to be shared throughout the landscape’s material, which ultimately reduces texture memory consumption.
Question 2: This would be better handled by using static meshes instead of landscapes. Some of the problems you would run into if you attempted to use a landscape would be the inaccuracies of collision, as stalagmites have steep vertical and aggressive changes. If you were to use World Position Offset and/or Displacement, that could further complicate things as your collision would need to bake the offset to be accurate.
Hope this helped!