Turning a stargate "off" is actually a fairly easy thing. Ba'al did it sometime in season 9-10. When SG-1 finally managed to retrieve those gates ba'al stole, they also managed to turn them back "on". If they know how to make a stargate link itself back into the network, it ain't too hard to figure out how to disconnect it. Doing much of anything more with the gate software, on the other hand, that'll probably be beyond them. The gates are linked through a complex subspace network. All they need to disable it, is to disable the transmitter/receiver function. For all we know, there could even be a means of doing so built into the DHD.

Having a DHD is a good idea. A DHD is far safer than anything custom-built by the Tau'ri. A lot of the mistakes they've made over the years when dialing from the SGC have been due to the lack of the classic safety protocols built into the DHD. We like that idea.