If I’m understanding correctly, it seems like the issue isn’t whether the latitudes are ordered, but whether they actually represent the southwest or northeast corners of the desired bounding box, which they did not in the non-working example you provided.
So I think to do what your are asking, instead of having parameters
nelat=0&nelng=52&swlat=40&swlng=-20
you would want parameters like
lat1=0&lng1=52&lat2=40&lng2=-20
that could be specified in any sequence and still get interpreted as the correct box. Does that seem about right?
Of course this feature may be useful only if we edit manually the coordinates. And the reason to edit manually the coordinates is the impossibility to draw a bounding box.
(I mean that an even better fix would be to allow drawing a bounding box on the map).
(Features are not independant from each other, the “product” is to be considered as a whole).