Are you sure the backlash problem has to do with the nuts and not the axial bearing or play on the guides? Did you try to move the table by pushing / pulling on the x-as motor? If you dont feel that much play as by pulling the table itself, the problem isn't the nut.
I suppose a bended axle has not much influence in accuracy in travel direction. My experience is a bended axle results in extra loads on the guides which will result in play onto these guides!
On my machine I already made twice new spindles, the fist of cheap threaded bars wich were bend to much, the second of grade A2 stainless (the iron store I was didn't had bars in 10.9 or 12.9 quality in stock at that moment) and new nuts from delrin. In the nuts, I made the second thread stage with a regular tool, the final stage with a part rest material of the SS spindle with a grinded cutting side. Result minimum backlash. If there will be to much backlash I know this will come from the nuts because I use preloaded axial needle bearings (see MF70 conversion) (Proxxon MF70 conversion). New new nuts will be made at minimum costst.
At this moment I only make small parts on my MF70 and use the overhanging weight of the stepper motor to eliminate the play on the guides. As I see how easy play becomes to these guides, I think its not a good idea to increase the travel to 40 or 50 cm.