IME, once they start to sag, it's over. the silly adjusting screw only pulls them tight in the middle and creates an unbearable, for me, longitudinal ridge that is usually worse than the sag. i usually just try to tilt them up in front so that the sag will position me near the back where i belong. i've had to discard a couple of them due to age. each cow's skin is different and the thinner the material (the B17 being the thinnest and the Professional being the thickest) the sooner the sag.

as far as the asymmetry is concerned, i'd just just live with it.

BTW, i've got eight bikes and four have Brooks saddles on them. some are 30-40 years old. i never put anything on them or adjust that silly screw. it just serves, IME, to hasten their demise.