Google-sheets – How to sort a column with date that is impeded within Texts

You can't really have "variables" in a spreadsheet; it's not a programming language. The values are contained in cells and can be referred to by location, possibly relative to location of other cells. If cells get rearranged so that the relations within rows or columns are lost, that is already a problem.

In your example, you want to refer to Count of Bananas in a way that is independent of sorting. The function vlookup does this nicely:

=vlookup("Banana", A:B, 2, False)

This means: get the value in column B that's in the row with "Banana".

Of course, if you sort column B but not column A, this is going to break; but then the entire logic of Fruit/Count columns is broken.

If you want to sort only the active sheet, it's simpler:

function onEdit() {
  var sheet = SpreadsheetApp.getActiveSheet();
  var range = sheet.getRange(3, 1, sheet.getLastRow() - 2, sheet.getLastColumn());
  range.sort({column: 1, ascending: false});
}

You had sheet.getLastRow() - 1 for the number of rows in the range, which doesn't seem correct if you start the range from the 3rd row. I put -2.