There is also the option of having an extra bar attached to the lifter which starts extending when the lifter reaches a certain point, helping with the self-righting (like what Ploughbot/Vertigo have/had) Whether you could get it into the weight limit, I do not know, guess it just depends what the mech is made out of.

In regard to your lifter attachment point - attaching it lower down will give it more power to lift with but I think with the way the Zeobot mechanism works, it may also increase the time it takes to achieve full lift. If you wanted to keep the attachment point as it is (thus keeping your lifters travel and time taken high) you could always bolt on a couple of lengths of box section steel (16mm squared stuff from the likes of Homebase would probably do) running the full length of the lifting arm - that will add some rigidity to the lifter plate.