1) If their support answered you that they don’t have that message in WooCommerce plugin files it only means that they don’t know their product and even don’t want to make a search before the answer to the customer. I have clearly explained to you what files of WooCommerce plugin include that text. Did you provide them this screenshot https://prnt.sc/oozg6y ? Did you check the screenshot by yourself to send proofs to support member?
You may download the WooCommerce plugin archive from the official site, extract it and open files mentioned on the screenshot and search for the text that you are talking about.
Also, you may enable any default theme and check the checkout page and you’ll get the same message.
Provide us with link to your topic to WooCommerce support.
2) Besides whilst and while are synonymous. http://prntscr.com/oqeu53 but if you want to change it you can use woocommerce translation files.
