I hope you don't mind me replying even though the post is addressed to Lucy, but I too have fears of teaching Grammar.
Like you, I always plan ahead for grammar lessons/sections and find it difficult if a student asks a grammar related question that I cannot give an in depth answer to. But I must say that I've learnt that if you try your best to answer the question and then back it up by saying "let me take a note of your question and do a little more research so that I can give you the best answer possible
" most students find that very satisfying because they know their teacher is interested in what they have to say and that they want to help their students progress.
Never be afraid to divert from your lesson plan, as you can never predict what problems/questions will arise. That's something I have learnt recently and once I realised it was ok, it has made things a lot easier. Just make sure you write yourself a little note as to why you diverted from the plan and what bits you haven't managed to complete. There's always next lesson
You mention that your students love multiple choice grammar questions but have difficulty when using the grammar point in a conversational activity. This is often the case as I have found that many students learn a grammar rule by heart so that they can easily identify it and write it down, but when using it in a speaking activity they tend to struggle as they cannot "see" the grammar (I hope I'm making sense). In this case it is always good to encourage more speaking and less writing once they have understood a grammar point.
Others may not agree with me, but these are just my views on what I have experienced and observed over the past 3 and a half years.
I hope my post has been of some use to you.