Having doubts is not a DF offence. Publicly talking about those doubts or suggesting alternative views openly can easily lead to being DF'ed.
Elders might seem like they want to understand you but you cannot trust anyone when they are acting in an "official" capacity. They can take your words and twist them very easily if they choose to do so.
So, it's probably OK to say "I just find it very difficult to get my head around things like the way the understanding of generations has changed".
But then to add "So I really find it difficult to trust that F&DS really know what they are talking about" can set the alarm bells going.
The obvious and less confrontational tack from the elders is "but you are sure Jehovah exists, you are sure his Kingdom is coming, aren't you? We just need faith in that and the rest does not matter." If you then start questioning that then again, the alarm bells start ringing.
Asking questions about the difference between "where" and "whom", birthdays, mediation etc. are all fine in principle and might be possible with a trusted friend but could very easily be a one way street to a JC if you confront a CO or arsey elder. Don't forget, they only have to form one and invite you. Who cares if you turn up or not. You're toast.
There is a reason why so many simply say "I'm depressed".