Soti Surf: set this app as default browser for all links ?
I don't have a Zebra TC75 to duplicate your problem or perform any tests. So the following are just general comments
If your TC75 Settings does not include any configurable option to change the device default browser, I think there is little chance you can do it with MobiControl. Actually over 95% of the hundreds of device models I encountered do not have such option in their Settings. Perhaps our friend Matt Dermody, who's more familiar with different Zebra models, can confirm with you if you can do that with Zebra MX on TC75.
As lockdown has been configured on your devices, your simplest solution maybe is to blacklist the pop-up activity in response to the comma key pressed being pressed. You can also try blacklisting or disabling "Settings".
From my experience, blacklisting Chrome should not be a problem in most cases, so I believe you probably have done it improperly, possibly with conflicting policies/settings that stop the device from getting the boot process completed.
Finally, you might need to consider upgrading both the v13.3.3454 MobiControl server and the v126.96.36.1994 device agent, as both are very old and there is no support for the new Bit Defender Antivirus/Malware engine nor include any bug fixes and new/enhanced features.