How to check Gmail Thread for Replies from Email - google-apps-script

Question

I am creating a basic CRM that needs to mark when a thread has been replied to.
I have created a script that can scan my inbox for threads from a list of emails in a sheet, check the last message in each thread and collect the .getFrom in order to see if I was the last to reply.
However, I can't figure out how to check if there has been a response from the person who's been contacted throughout the whole thread.
Here's the script that checks for the last message. (It's an extract of a larger script in case any references are missing):
Example Sheet
function UpdateStatus() {
// Connect to our active sheet and collect all of our email addresses in column G
var sheet = SpreadsheetApp.getActiveSheet();
var totalRows = sheet.getLastRow();
var range = sheet.getRange(2, COLUMN_WITH_EMAIL_ADDRESSES, totalRows, 1);
var emails = range.getValues();
// Attempt to iterate through 100 times (although we'll timeout before this)
for (var cntr = 0; cntr<100; cntr++ ) {
// If we've reached the end of our last, wrap to the front
if (lastRowProcessed >= totalRows) lastRowProcessed = 1;
// Increment the row we're processing
var currentRow = lastRowProcessed+1;
// Get the email address from the current row
var email = emails[currentRow-2][0];
// If the email address field is empty, skip to the next row
if (!email) {
lastRowProcessed = currentRow;
cache.put("lastRow", currentRow, 60*60*24);
continue;
}
// Look for all threads from me to this person
var threads = GmailApp.search('from:me to:'+email);
// If there are no threads, I haven't emailed them before
if (threads.length == 0) {
// Update the spreadsheet row to show we've never emailed
var range = sheet.getRange(currentRow,13, 1, 4 ).setValues([["NEVER", "", "", ""]] );
// And carry on
lastRowProcessed = currentRow;
cache.put("lastRow", currentRow, 60*60*24); // cache for 25 minutes
continue;
}
// Beyond a reasonable doubt
var latestDate = new Date(1970, 1, 1);
var starredMsg = "";
var iReplied = ""
// Iterate through each of the message threads returned from our search
for (var thread in threads) {
// Grab the last message date for this thread
var threadDate = threads[thread].getLastMessageDate();
// If this is the latest thread we've seen so far, make note!
if (threadDate > latestDate) {
latestDate = threadDate;
// Check to see if we starred the message (we may be back to overwrite this)
if (threads[thread].hasStarredMessages()) {
starredMsg = "★";
} else {
starredMsg = "";
}
// Open the thread to get messages
var messages = threads[thread].getMessages();
// See who was the last to speak
var lastMsg = messages[messages.length-1];
var lastMsgFrom = lastMsg.getFrom();
// Use regex so we can make our search case insensitive
var re = new RegExp(email,"i");
// If we can find their email address in the email address from the last message, they spoke last
// (we may be back to overwrite this)
if (lastMsgFrom.search(re) >= 0) {
iReplied = "NO";
} else {
iReplied = "YES";
}
}