You can go ahead with Napoleon, just make sure you don't significantly overlap your major work with content covered in the syllabus. If you're teacher has looked at it and gave it the go ahead its fine - remember the major work it marked internally by your teacher, so their advice is always the best advice (in most cases). If you are extremely worried, you can change your topic. I do ancient History, and King Arthur is definitely not an option at all the Ancient Syllabus (King Arthur, originating more so from a Medieval Time Period rather than Ancient Time Period anyway). And yes, King Arthur has been done numerous times in the past by History extension students, as there is just so much controversy with Historians about his life, his achievements, his ideas, his origins, etc, etc. n_n There's more than likely some past major works in the resource section of this site if you want some more inspiration, or even if you look at some old HTA article publishments they usually publish some great Extension Projects in their magazine, there's a link to their article database I'll go find it after I post this (though, some articles to cost money to order and purchase).
The only issue I see with changing your topic, is the time restriction you're leaving yourself to. My school's projects are due at the start of next term, so that leaves you roughly with a term do complete the major (if your school sticks to this criteria), though I do know people have completed the major work days before the due date and have scored good marks for it (though I personally wouldn't leave it to this stage!). I know quite a few people who are on the final processes of their major works and are onto completion.
So it's really up to. Again, talk it over with your teacher, talk with them about maybe changing topics, and see what their opinion on it is. I don't see too much of an issue with you doing your Major Work on Napoleon as you can take on a completely different approach to your major rather than follow in line with the Extension syllabus (so the areas your teacher mentioned would be great!). But really it's up to you, either option isn't too horrible, as long as you're a hard worker and are determined to get it done. And also, that you don't procrastinate or buckle under stress! If you do, try and get this done as soon as possible.
Remember, with the project being internally marked the best advice is your teachers, so always consult them with any issues or problems you may be having with the project.
