Miui v6 5.8.14

Hi,

I have MIUI V6 5.8.14 installed on my HTC One X +.

I have 4 questions.

1) What is the difference between V6 5.8.14 & 7 5.8.21?
There are improvements necessary for7 5.8.21 and V6 5.8.14 is the final version of its kind?

2) I found 'Sim toolkit' in app list, but I don't know were I can open this application?

3) Is there a solution to reduce the volume more? Even the lowest volume is to high for me, especially when I use the alarm.

4) OTA 5.8.28 is for upgrading from 5.8.21 I think. It is also necessary to go to bootloader or is there a different way to do the upgrade?

Thanks in advance.