Google Sheets – Set Header Row Range as Array of Header Names

That's currently not possible in Google Apps Script. See this enhancement request in the Google Apps Script issue tracker, to make that possible:

• issue 4245

As mentioned in the issue itself:

To subsequent readers: If you are also interested in this requested feature, please click the star next to the issue number.

A custom function will always receive either a rectangular array or a scalar value (number, string, boolean, or datetime) as each of its arguments. So the conversion from rectangular to jagged array has to happen inside the function. For example, by calling jagged(arr) on the input, where jagged is the following function:

function jagged(arr) {
  var rowLength = arr.map(function(row) {
    return row.reduce(function (a, b, i) {
      return b === '' ? a : i+1;
    }, 0);
  });
  return arr.map(function(row, i) {
    return row.slice(0, rowLength[i]);
  });
}

This function first computes the actual length of each row and then truncates the rows accordingly.

I emphasize that the call to jagged needs to happen from within the custom function that will use it, like so:

function myFunction (arr) {
  jarr = jagged(arr); 
  // proceed to work with the jagged array
}  

Putting =myFunction(jagged(A1:F3)) in a spreadsheet cell won't do it: the Sheets will automatically inflate the return values of jagged to a rectangular array before passing them to myFunction.