Sods law isn't it.
As I have stated previously TMPIN2 normally points towards the Northbridge but of course, as we are dealing with computers, that doesn't have to apply with your motherboard......Aaargh!
There are a few possible scenarios here;
1. The sensor is reporting Northbridge Temps but you would need to check with HW Monitor's support for confirmation of this.
2. The sensor is not working properly and is fixed at a constant temp which is why there is no variation when you have tried to adjust the airflow through the case or when you removed the side panel. This is really not unusual and a lot of boards seem to suffer from this
3. There isn't actually a sensor for TMPIN2 on your motherboard and HW Monitor is reporting an erroneous figure. But, as you have already discovered, getting confirmation of this from Gigabyte isn't going to be easy.