Are they bright enough to realise that?As Franc said, it is not clear atm.
I guess that if you buy the box yourself, Sky will not care. You did not get any interactive discount on that.
One thing is sure: They can't take away the box you bought yourself. They could turn off the viewing card, but they would really be stupid if they did this.
Twin LNB is fine for just one Sky+ box.
The phone line issue is not really clear at the moment but I can confirm I have done one this week in a caravan and we did not need the phone line to pair the card when paired to second hand Sky+ box.