While the individual GB members almost certainly don't agree on all points of doctrine (otherwise, where would the "new light" come from?), they do believe the most important doctrine of the WTS/JW religion. Namely, that the GB are the representatives of the FDS of Matt. 24 and that they lead God's Organization.
I think that the GB (like most JWs) see doctrine as secondary. The most important thing is that they lead God's Organization. If you believe this, you can rationalize anything. Just look at the Bible for examples of "God's Organization" being a total train-wreck. The kings of Israel were worshiping idols in the temple, sacrificing their kids to false gods, etc. Thus, any errors/changes/etc. in modern JW doctrine are minor and inconsequential compared to the conduct of ancient Israel. Of course, this line of reasoning only works if you accept the premise that the GB/WTS is "God's Organization." And I think the GB members accept that premise.
So, to sum up, I think the GB believes they are who they claim to be--but don't necessarily believe every bit of current JW doctrine. And I doubt they see much of a problem with that.