Selecting the dense point cloud inside a region:

[Ingsayyad]

I want to select a dense point cloud inside a region (to delete it /to replace it/to merge two point clouds). Currently, I can perform these tasks by rectangular/ free-form selection, but it delivers undesirable results.

[Bastiaan Veenstra]

Wouldn't it be possible to select that part of the model using the sideview (3 on your numpab) and switching to orthographic view after that (5 on your numpad)
You should be able to select just that part you are showing in the screenshot.