Lock-range code for Google Scripts/Sheets

This code locks a range and adds a timestamp. More details below. I'd like to learn how to make this code more efficient (minimize code/variables and reduce redundancy). Any thoughts on the areas I can improve?

function lockEdits(e) {

  // delcare initial col variable
  var colCheck = e.range.getLastColumn();

  // exit function if the col edited was not 11, 17, 20
      if (colCheck != 11 && colCheck != 17 && colCheck != 20) {
        return;
      }

  //  // delcare remaining variables
  var ss = e.source.getActiveSheet();
  var thisRow = e.range.getRow();
  var rngHeight = e.range.getHeight();
  var email = Session.getActiveUser().getEmail();
  var owners = ["[email protected]", "[email protected]"];
  var checkEmpty = ss.getRange(e.range.getRow(), colCheck).getValue();
  var rejectCheck = ss.getRange(e.range.getRow(), 17).getValue();

  // if change in col 11, enter user email and timestamp, then protect range
  if (colCheck == 11 && checkEmpty !== '') {
    var protection = ss.getRange(thisRow, 2, rngHeight, 10).protect().setDescription('Lock Range:');
    var nEmail = ss.getRange(thisRow, 21, rngHeight, 1);
    var nStamp = ss.getRange(thisRow, 22, rngHeight, 1);
    nEmail.setValue(email); // print email
    nStamp.setValue(new Date()); // print timestamp
    SpreadsheetApp.flush();
    protection.removeEditors(protection.getEditors()); // protect range
    if (protection.canDomainEdit()) {
      protection.setDomainEdit(false);
    }
    protection.addEditors(owners);
    SpreadsheetApp.flush();

    // if change in col 20, enter email and timestamp, then protect range
  } else if (colCheck == 20 && checkEmpty !== '') {
    var protection = ss.getRange(thisRow, 17, rngHeight, 4).protect().setDescription('Lock Range:');
    var vEmail = ss.getRange(thisRow, 23, rngHeight, 1);
    var vStamp = ss.getRange(thisRow, 24, rngHeight, 1);
    vEmail.setValue(email); // print email
    vStamp.setValue(new Date()); // print timestamp
    SpreadsheetApp.flush();
    protection.removeEditors(protection.getEditors()); // protect range
    if (protection.canDomainEdit()) {
      protection.setDomainEdit(false);
    }
    protection.addEditors(owners);
    SpreadsheetApp.flush();

    // if rejection in col 17, enter email and timestamp
  } else if (colCheck == 17 && rejectCheck == "Rejected") {
    var vEmail = ss.getRange(thisRow, 23, rngHeight, 1);
    var vStamp = ss.getRange(thisRow, 24, rngHeight, 1);
    vEmail.setValue(email); // print email
    vStamp.setValue(new Date()); // print timestamp
    SpreadsheetApp.flush();
  }
}

So a few notes on what this does:

This script is set up as an onedit trigger and this sheet is shared with multiple users.

• If a user edits a cell in column K (11), then lock that row/range from columns B-K. Then also add the users email in column U and a timestamp in column V.
• If a user edits a cell in column T (20), then lock that row/range from columns Q-T. Then also add the users email in column W and a timestamp in column X.
• If a user edits a cell in column Q (17) to "Rejected", then just add the users email in column W and a timestamp in column X.

This works as-is, I'm just not sure this is the most efficient way to do it and I'm hoping those of you with more knowledge can help me fine-tune this a bit.

Let me know if any other info would be helpful in sorting this out!