Like I said in another thread, the point, as I see it, is to distil your mind rather than develop one-pointed-concentration (a term I dislike, btw).
Consider the problem of this term: one-pointed upon what? What can occur in the mind as a single point? A perception? Alas, a perception does not occur spontaneously but instead depends on an interaction of the body with itself or the world (or of the mind with itself and the world). If you attempt one-pointedness upon that perception then its supports are cut away from the attention of the mind, and so the perception ceases. Likewise if you attempt one-pointedness upon a feeling: you cut away its supporting factors by denying them attention, and the feelings falters and ceases. Likewise a thought will be cut from its supports if you attempt one-pointedness upon it, and it too will cease.
Instead you determine what aspect of your bodily process is most easily engaged by your attention, then you turn your attention to it (maybe the whole body breathing as it breaths, or maybe only the air on the nostril, top of lip, or maybe the flexing of the muscles in the sides of the chest - or anything else connected with the breath). From that attention there will arise a steady 'stream' of perceptions related to the chosen process. There will also occur feelings and thoughts and such. Not one of them can form a single pinpoint for one-pointedness to rest upon. Even your attention may waver, and the perceptions will alter in their character. If that has happened unintentionally, then return your attention to where it was.
In this way you establish the 'theme' of your mind in a way that is wholesome, and you do it intentionally. You consider the quality of that theme and maintain it. Within that theme, and within all themes whether wholesome or not, the first three satipatthana are contained automatically. If you are establishing your theme intentionally, maintaining your theme intentionally, and assessing the qualities of your theme, you are necessarily drawing in the fourth satipatthana.
And so the whole thing is complete, and simply needs to be developed often in order to reach refinement.
A last note about the scope of attention. By scope of attention I mean the number of perceptions that arise in a given time. If you choose the muscles of the chest, fewer perceptions will arise in a given time period. If choose the whole body as it breaths, then more perceptions will arise in a given time period. But in both cases the perceptions will include the perception of the breath and be unified as a theme around the perception of the breath (which is why this remains anapanasati and doesn't change into some other manner of meditation).
As always, I am not certain about my clarity or whether I've said anything helpful. But at least I can offer good will. Metta.