Continuing the series about three overgrown lithops and an attempt to save them and get them into proper shape, here is the most recent development.
The lithops were shrinking as expected, however I fear I noticed way too late, that the tallest one had grown the new leaves inside to a size that is definitely too long for them! Actually, I wanted to cut it open before this happens, even though I understand that the'd be immature at that time, so it might have been bad either way. Now that I noticed, I've ripped the old leaves open to get some light upon the new ones and prevent them from growing even longer. I'm pretty sure I was too late but we'll see. I was afraid to miss the "point of no return" with the other plants so I've ripped another one as well. The third one doesn't show any signs of new leaves though, which is even more worrisome. Oh well, it is an experiment so we'll just have to wait and see. I guess, if they survive, it will be another year until they look normal.
For your future reference, if you buy cucumber shaped lithops, prepare to spend 2 years correcting them - in a best case scenario! It's really not worth it. In 2 years you can grow your own perfect little lithops kids from seed ;)