I'm sorry you're facing yet more trouble. I kind of freaked out myself when Kulu got coccidia because of loosing my first chinchilla but the antibiotics did their job and everything cleared up. The 10 day course seemed to take forever. It makes it way easier when you medicate or syringe feed with two people and have everything set up in advance, the medications already dosed, damp rags sitting ready on the desk etc. to make things go as quickly and smoothly as possible.
I think sometimes these kinds of things don't have any good explanation but usually it's eye infections that you hear more about. I don't know that much about sepiolite. I've always used Blue Cloud.
Here's the info on the oxbow critical care dosages:
http://www.oxbowanimalhealth.com/resources/dyn/files/98781z8d46767/_fn/2008+Critical+Care+feeding+chart+front+only.pdfSo based on my knowledge of this (and you may want to double check my figures) if your chinchilla's healthy weight is 500g it calls for 1.7tbsp, and 1kg it calls for 2.9tbsp so in between that is 1.8tbsp and that converts to about 27cc. I'm not sure what you're measuring with but you'll be good if you're in the ballpark.
So that's the total amount for one day for an animal that is if your pet is not eating anything else. So unless your chinchilla goes completely off its food start by cutting that in half and you get about 14cc. Mix that with two parts water (add 28cc) and break that into two feedings (let's just round to 14) so each feeding is about 14cc. You can adjust the amount of water larger or smaller to get the consistency you want within reason.
I do recommend calling your vet and asking him about what dose and mixture he would recommend and let him know you are having a problem. I am not a vet and cannot guarantee my advice is the best route.
If your chinchilla goes completely off its food notify your veterinarian and get further instructions. These animals will do better with a more constant rate through their system and you may want to feed three or four smaller feedings per day as evenly spaced across 24 hours as reasonably possible.
I do also recommend removing apple sticks from the cage if she is eating them and leave the pumice and regular pine toys. I know some chinchillas will do this to the exclusion of their normal diet to certain kinds of sticks. This may improve her appetite.
As far as loosing your pet, and that is pure speculation and I don't see that happening because she is otherwise healthy, I've seen many cases where the cagemate does just fine. I wouldn't worry too much about it. The worst cases are usually when the chins are a breeding pair.