Haven't looked at your sketches so don't know if this suggestion would work because you're using a zip.
If the overshoe part was removable (e.g. velcroed) although usually left in place on the end of the overtrouser leg you could get round the problems of separate use and different rates of wear. You could then sell replacement overshoes separately. Depending on the way you positioned the velcro and the height of the overshoe 'boot' you could market the 'perfect fit' possibilities and maybe save a bit on the number of different leg-lengths you would have to produce.
I think you'd have to replace your proposed zip with velcro, too, but that might have cost benefits anyway.
Let me know if you decide to knock up some prototypes and I'll send you my inside leg measurements