Hi Byron Nice,
Welcome post in Microsoft Community.
I'm Mosken, I do apologize for the inconvenience that you're experiencing right now, let me help you sort things out.
According to your description, I knew that your users can edit the source code when using older IE browsers, but only the non-editable version of the source code can be opened in the Edge browser. If my understanding is wrong, please correct me in the reply.
You can use the "Ctrl + Shift + C" shortcut or right-click on the blank space of the Edge browser page > "Inspect*" (bottom), then open the source code editable version of the web page*, and then edit it, as shown in the following figure Show.
The above steps are provided based on the information I have learned so far, I wish that will be helpful to you. If your problem still cannot be solved by these steps, please tell me more specific situation in the next reply. I will continue to assist you in solving the problem.
Best Regards,
Mosken - MSFT | Microsoft Community Support Specialist