validity cells infest spreadsheet

Trying to find out why 2 programmed validity cells no longer drop menu. Checking cell range - OK. Repeating creation of validity for desired cell range. Result: Still no drop-down.
Focusing on other, empty cells. Many have a useless drop-down arrow.
Checking cells with content (numbers) - some of these too have drop-down arrow.
Checked some of the infested drop-downs: They all (?) seem to point to the same cell range which is the one not working (no drop-down) in the intentionally programmed 2 cells.

Emphasis: While a validity cell could be copied accidentally into empty area it is highly unlikely the same would happen into a range with numbers content - because numbers would be replaced which is easily to see and undo.

  1. Why can desired cell not accept accept drop-down? Cell range shown is correct.

  2. How did lots of other cells get invested with undesired validity drop-down?

  3. Search method for validity cells?

  4. Prevention of such accidents?

Follow in question/292192/.