Populating A List Based On Four Toher Lists

I have Columns A:D with a heading at the top. the user can then type organisation names under the correct column. There is a maximum of 30 entries (rows 2:32).

Problem One:
Column E is a hidden column and I need cells E2 and downwards to be a list which gets populated by ALL the organisation names inserted into $A$2:$D$32. I don't want there to be any blank cells until the last entry in column E (if that makes sense). how to populate this list?

Problem Two:
At another part of the worksheet, I have five cell where the user has to select from a list. The five cells correspond to the lists in columns A:E. Is there any way to avoid having blank options in a list using data validation (say if the user has not entered 30 organisations into one of the columns)?