Your TOC works fine on my machine. I seldom use the TOC inside a content control. It slows things down. If you instead insert a Custom (not Manual) Table of Contents you do not get the Content Control.

All I see for your button is a graphic. However, I believe that an ActiveX button would need a .docm format document to run.

Again, the button should not be necessary. Alt+Left Arrow returns the TOC. You could also add a Cross-Reference to your Table of Contents acting as a hyperlink if your really needed it. That would not have to use ActiveX.

On this second computer where it does not work, what happens if you open Word in Safe mode?
Open Office apps in safe mode on a Windows PC - Microsoft Office