Here are a few thoughts to start with:
(1) If you see the gap but can’t really articulate it, I would encourage you to spend a bit more time working on being able to articulate it- the extra time spent up front figuring out the stimulus really pays off when going through the answer choices. Out of the question types that you listed, I think sufficient assumption questions are probably the most important to always try to articulate how to fill in the gap before moving on to the answer choices.
(2) This is one of the things that is important to cover during review- not just understanding why the correct answer is correct and why the incorrect answers are incorrect, but also going back to the stimulus and seeing how you could have articulated the gap before going into the answer choices.
(3) The correct answer on necessary assumption questions tends to use weaker language “it is possible that once in a while smoking isn’t the best thing for you” in comparison with the correct answer on sufficient assumption questions, which tend to use stronger language “smoking is always bad for you”. So, on necessary assumption questions, some answer choices are more likely to be incorrect simply on the basis of the language being too strong, and on sufficient assumption questions, some answer choices are more likely to be incorrect simply on the basis of the language being too weak. While this isn’t a foolproof method, it can help.
I know this doesn’t come close to answering all of your questions, hope it helps a little bit 🙂