It's a natural progression of difficulty level. The IR being the most difficult (in theory) and each of the previous courses has content that relates very neatly into the next. That said there's no reason why you can't do it a different way.
As for the exams, you need to complete them all within 18 months of your first pass. Better to get them all out of the way, also you are in exam mode which means you are focussed on just that, they're not hard just a lot of work all the distance courses are geared to doing them all in 2 or 3 blocks and they have success in doing so.