Hi, please keep asking questions, as the idea of this site is to help chipmunk owners in the UK. Although there are not many of members yet, there are already plenty of people reading the forum. I still feel i have lots to learn about chipmunks myself, and I hope that others will chip in with their experiences and tips, as I'm no expert.
When I first got my chipmunks my place was carpeted throughout. But the place became so hard to clean because of the chippies ramming seeds right down in to the carpet. So I've now got wood floors instead, very much easier to clean, but more chance of a chipmunk having a hard fall. And they do sometimes when they miss their footing. I've seen mine land on the floor from up to eight feet. Although this shakes them up, they are very tough, and within a few seconds they race off. Except for one time that is. When Snowey was a baby he fell on to the floor and he hurt his back leg. I didn't know if it was broken, the vets was shut, so I wrapped in some fleece and stayed up with him all night. The next morning he still had a very slight limp but that soon went, and he's fine now. Now I don't think he had broken his leg, or perhaps because he was so young with soft bones he was able to heal so quick. Ever since that night Snowy never sleeps more that about four foot from me in a hammock hanging from the ceiling. I think it was then he realised his mother wasn't such a good mum after all. But if it happened again I would take him to the vets the next day to have him checked up.
I'm concerned about Zippy, because I think he's been like it for a while from what you say. With my recent experience with Sue who was in such pain I know now that when in pain they hardly eat, spend all their time in the nest, but don't make any noises not even when you handle them. But if he tries to bite you when you handle him, and he doesn't normally, then I think you can take it he's in pain. I think if Zippy were mine I would be off to the vets now, see if he can have his leg x-rayed, and if he is in pain see what your vet can do to help him. You maybe surprised what skills and modern techniques vets have now, probably not a splint, but the vet might have some good advice at least on how to boost Zippys weight.
Let me know how it turns out.