And why do we have to classify everything and fit it into our boxes or else it's invalid?  Why do we have to classify Satan?  He is a being, as we are, he is spirit, not bound by flesh although able to take on the form of flesh, he was created by God just as we and he has power (if we give it to him) just as we have power if people give it to us.  God is STILL over him as we see in Job where Satan asks God permission for things.  That one instance you refer to where one account says David's account of the people was the Lord's hand and one was the enemy shows this perfectly.  Satan goes always before God's throne after having roamed the earth looking for a "case" he can present before a God he knows is just.  Satan rightly saw the pride in David's heart and made a case to incite him to number the Israelites.  (This is why Satan is called the accuser because he constantly is bringing accusations before the Lord against us -- praise God for Jesus, as now we can satisfy God's justice with the atonement and silence our accuser; sadly too many don't understand this principle and so we see many Christians viewing Satan as one whom has more power than he does, and they live in defeat).    

So some may rightly say God in his authority and perfect sovereignty brought David to make the census (it wasn't God who caused the sin in David's heart, but rather God who saw the sin of pride in his heart and allowed the fruit of that sin to come forth to show David what was in his own heart) and some may rightly say that Satan was the one who brought it on (as it was he who tempted the pride in David to bring it to manifest fruit).  

Thanks for reading and for exchanging ideas with love and respect!