Right now it is all just a possibility. They both have certain visions for the North's future, but I find it hard to bet on any of them. Radovid's approach to stability being a complete elimination of one variable (the mages) and thus reducing a number of potential problems makes sense, but only if the North can survive the next invasion. Carduin was ready to play ball, so there was no need to start the massacres.The Knights turned onto Foltest, and were exiled from Temeria. Right now they obey Radovid, but could he really control them? Roche agrees to free Anais, but there is no real incentive to give her to Temeria. I hate to sound like Saren on the Council

, but I am not impressed with Radovid. We'll see how the events will develop in TW3.
Radovid isn't trying to eliminate the Mages, he destroys the Lodge and ends any chance of them being a political power in the north, the Nilfgaardian inspired massacre is just a side effect and one that all the powerful Mages avoid as Roche or Iorveth tell you when returning to Loc Muinne. The fact that Radovid was conspiring with Carduin points to the fact that they were both content with what would happen at Loc Muinne, the Mage and his faction gain power, the Lodge is destroyed and the hopes of a Conclave are dashed. Radovid simply took control of a faction of the Mages, as control is needed, the Lodge's silly destruction of Aedirn's military potential while Nilfgaard is waiting in the south is a clear indication of this. They handed the country over to the Black One's just as much as Letho did.
To control the Order I believe require a more indepth explanation which i'll split into points below.
1. Have a hand in creating them: Redanian intelligence created and supposedly sponsored the Order for a very specific purpose, as a counter weight to the Mages, and presumably there are still agents within the Order that report directly to the Redanians. Thus we have an "in" with them.
2. Destroy their leadership: This is simple, Jacques must be slain and with a few words in Foltest's ear in Vizima and the help of a well payed Witcher, Radovid accomplishes this. This was obviously one of the reasons why he was in Temeria in the first game, uniting the two countries is a long term goal, securing his own throne is an immediate one, and a well trained army of Zealots helps immensely.
3. Save them: Jacque DeAldersburg attempted a coup, and his men are as guilty as him, they have sinned against a divinely appointed monarch and commited blackest treason. With their messianic leader slain and cold reality setting in they are looking at an evetual dissolution of their Order and to dancing the gallows jig. In steps Radovid to take them off Foltest's hand, give them a chance at redemption, stop any further bloodshed and save the day. Very bloody smart.
4. Replace the Brass: Radovid places his own men in charge, promotes those who are allready in service to the Redanian intelligence service and basically makes it his own organisation. If Siegfried is in charge this will still happen but Radovid will trust the Knight, who wouldn't? Siegfried for all of his valiance and honour is a straight forward man, and we know that Radovid also plays square with his allies, he talks straight as he does with the Witcher and Roche, and keeps his word. I get the feeling Siegfried will thrive under such a man.
5. Maintain the image: Radovid will have to act as the straight forward, plain speaking, open and honest man he's acted as in both games, and pursue the righteous causes that the Order espouses. The elimination of the Lodge is just such a righteous action, these are mass murderers and king slayers, they typify all that the peasantry and the nobility fear that Mages may become and rooting them out and punishing them is a holy duty. Thus through his actions Radovid confirms himself as a rightful ruler and a truly divinely appointed monarch when he brings the Order to Loc Muinne.
6. Support their religion: This we have yet to see, whether the Eternal Flame has flourished in Redania, we'll probably see when in Novigrad one way or the other.
Personally i'm impressed both with Radovid's actions towards Geralt, his plans, his mastery of the game and his daring. The last more than anything, a good king must seize the day and roll the dice, take the chances and trust to his actions rather than sit and do nothing when action is called for, because such an idler will be soon overtaken by events and deposed. From his introduction in Vizima to the denouement of Assassins of Kings Radovid has shown a clear and detrmined pursuit of his goals, and been incredibly pro-active in securing them. He had to destroy the Lodge, Philippa and her ladies proved that when they slew his father, and cemented it with their largely successful coup in Aedirn. His only blind spot may be in working with Nilfgaard, but so far that has not turned against him, only aided his cause.
I'm extremely interested in what happens come the third game, and what role Radovid plays in it. I can't seem to remember who Saren is, what council did he serve on in Redania, was he part of Philippa and Djikstra's?