SEPM Upgrade to 12.1 RU6 MP8

Hi,

I have SEPM 12.1.6 (RU6) - 12.1.6168.6000 with clients of different versions (like 12.1 RU6 MP8, MP4 etc) I want to upgrade SEPM to 12.16. (RU6 - MP8) 12.1.7266.6800.

My questions are what precautions (other than backup the database) should consider before to make an inplace upgrade.

1. Do I need to withdraw policies - if not, after upgrade will the policies be intact?

2. What about the groups? 

3. Do I need to backup server private key?

4. How can I prevent restarting of SEPM PC after the upgrade?

5. If (worst case) upgrade failed then, how can I restore SEPM to the one before to the upgrade?

Thanks,