Brendan had something to do with Taj turning on him, he didn't offer any insurance to the others at merge that their plans were still intact. They were on his turf, and his tribe had the numbers, it was up to him to approach and say that things were still golden.
As for Stephen, I don't trust him, but until he actually crosses JT or Taj, he's still good. Hell, I feel the same way about JT. He's no better in my eyes. The thing is, it's easy to contemplate stealing something so valuable and using it to your own benefiit. Unless one of them actually does it, I'll give them the benefit of the doubt.
I think people may be underestimating Taj. So far she's played that idol just right. It has bought her the security of the guys having total faith that they not only have her trust, but that they can control her vote. This may or may not be true. Time will tell if she is following...or setting herself up to be in a better position in the end. There are many ways to use an idol. So far the way she has chosen to use it is working out in her favor. Take the backseat. Let the guys take the hits for making moves. She has time to reveal herself as a true gamer in the end, it's way too soon now when things are going her way anyway. Let yourself fall further down the list of those who need to go, therefore making sure that many leave before you are under any real threat to leave. That's the game she seems to be playing. Hopefully it works out for her. The idol will divide them if she brings it up. Leave it alone for now. Let it divide the boys should it come down to that, leaving them both running to her for support.