The deprecated notice from your 3rd party extension called "cegcore2" library files is why the display after saving is broken. It breaks the JSON response. Fix that or turn off error reporting in Joomla global configuration to suppress it and the JSON response will be fine.
As for the remaining issues it looks like I'll need to add custom handling to CB Field Group like what was done with image fields for when a new row is inserted. Have added a bug ticket to fix that. I cannot fix the JSON response issue as that's caused by a problem specifically on your site.
forge.joomlapolis.com/issues/8684
I however don't recommend using a field group field for what you're doing. I suggest using CB Gallery instead and create a Gallery Field for holding your file uploads or uploads of any kind where you need to allow multiple in a single field.