As for the update. You will need to make sure your php version is 5.5. As of version 1.8.2 this is a requirement. Previous php versions will cause issues.
As for the second one. You may want to check into the Paid Membership plugin. I understand that it also requires a payment gateway to work. I don't use it so I can't tell you much about it.
As for the third one, I think that has been asked before, but I don't think there was a resolution other than custom code modification, or you can use a responsive theme, and use it's capabilities for mobile view.