About 10% ~ 20% of JWs believe "everything single thing" that they are taught from the GB in NY.
The rest have doubts in one way or another. I'm sure that many of us do/did.
Most JWs believe what the entire Bible teaches, but not everything what the GB teaches. If the R&F aren't sure about an issue, they may opt to try and understand the reasoning from the GB. From that point, they are willing to accept such teachings. However, some may have doubts and will still harbor these.
For example, I mentioned to several in the congregation about Jesus being our Mediator. They agree because that is what the Bible says. When I told them and directed them to the WT articles and Insight Book saying that Jesus was mediator only for the annointed, they acknowledged that fact as WTS reasoning, but they still didn't believe that Jesus wasn't their personal mediator and Savior.
There are many issues and a variety of teachings in WTS theology. If you were to lump them all together, honestly you will only find 10%-20% of all JWs who believe every single tidbit of it. If you were to filter out any who don't believe, then you'll have various ones who can't line up the Bible's reasoning with the WTS.
They only doctrines that almost every single JW believes in is the non-Trinitarian view, no Hellfire, no immortal soul and living forever either in heaven or on earth. Funny, isn't it? These teachings that haven't changed since the conception of this religion are the teachings that most Christian religions don't accept. The core doctrines of what JWs don't believe in are the ones that never change. All the other "understandings" are just that, understanding. Not truth. Truth and understanding are two different things.
Alligator Wisdom (aka Brother NOT Exerting Vigorously)