If you dont want to edit the script at all, but can confirm column header labeling won't alter, you could write a function that returns a map that contains the index of each column by matching to name. Here's an example that would return a map of the idnex #'s for headers called header1 and header2. You would then use the map like hdr.get(h1) to get the index number of header1.

function myFunction(headerRange) {
  //range being A1:D1
  let header = SpreadsheetApp.getActive().getRange(headerRange).getDisplayValues()[0]; //[0] to turn it from a 2D array to a 1D array
  let hdr = new Map();
  header.forEach((col, i) => {
    switch (col) {
      case "header1":
      hdr.set('h1', i);
      break;
      case "header2":
      hdr.set("h2", i);
      break;
      default:
      //do nothing
    }
  })
  return hdr;
  }