There's not much chance that the DSTT will break your NDS (i.e. blow the fuse). It happens very rarely, and there's not much you can do to prevent it (if you've had your NDS for under a year, the damage would be covered by the warranty). Far more frequently, your DSTT simply won't work due to not making proper contact with the NDS. There's not much you can do if that happens other than stuff some paper or cardboard in with your DSTT to try to help push it towards the NDS contacts. If you have a contact problem, you're better off just returning it for a new one.
If you have a working DSTT, its life span should be many years.
Fakes work pretty much just like real DSTT's other than being susceptible to the fake-killer firmware. There is a program you can use to restore a "killed" fake DSTT, so it's not that big a deal. Of course, you should always use the fake-killer-disabled firmware to avoid having your fake killed. So bottom line -- fakes aren't so bad, but personally I'd still return it for a new one. Usually if you receive a fake, DX will send you a new one and won't require you to send the fake back.
-Bri