Anapanasati - are the 16 considered progressive sequence of steps?
Try not to overthink it. There is a clear path of progressive steps or progress, yes, but for each meditator it varies somewhat as they each have varied conditions. We don't control what arises, so we can't expect to watch it unfold like a textbook.

The stages or steps occur naturally, without us having to force or think about it, as long as you are doing the one basic thing correctly: observe with wise attention whatever arises, without judgement. 

You'll notice each grouping of steps follows a pattern of prescriptive action - this pattern of treatment is the important thing. Apply this pattern to whatever arises, and the Path will unfold before you, eventually without "effort". At first we start with the coarse, and naturally go sublter and subtler as our minds get more still and concentrated. The next thing to contemplate will be waiting for you, and arise on its own. Don't try to seek out an expected experience. Just observe whatever it is as it arises, watch it as it disappears, relaxed, without clinging. Its shockingly simple when it finally clicks. No need to wonder if you're at stage 13 or should I now contemplate the next thing on the to-do list or anything like that, that will just stir your mind up.

The breath is simply our home base, something we can always return to if we wander. Sticking with the breath trains our focus, trains our concentration, trains our resistance, trains us to not cling or push away. It is important to remember that the training of these skills are the key, not what object you use. 

Master the method, and the Royal Road to Nibbana will appear. Good luck. May we all become truly free.