First up, with subject order, I'd recommend doing it in recommended order as far as possible by the study plan on the MSE website. Whilst I haven't done the Masters yet, I've had similar issues with starting my Bachelors with a Mech Major in Semester 2. Not doing so can make certain subjects much harder than they need to be as though a subject may not be a pre-req per se, there may be a certain amount of assumed knowledge that is carried over, or it could potentially extend your course beyond the 3 year mark due to subjects not being available both semesters.
With difficulty, Level 2s and 3s are effectively Bachelors subjects, whereas the Level 9s are Masters. So expect them to be less intensive, and easier in terms of content. However, from friends who have gone through the Bachelors and are now doing the Masters, one thing all of them have said is that the Bachelors subjects for Engineering at least are generally horribly taught in comparison to the Masters subjects, so just don't neglect them is all I'd say.