Firstly, I am so sorry to hear about your trouble with this adorable baby bunny!
Being a breeder myself, I'm strongly against bottle feeding as I feel nature will take its best course.
Through trial and error I've found that giving the runt extra assisted feedings with mama bun 2x per day (helping it find and attach to a teat, while momma bun is on her back) has helped runts survive and thrive - whereas bottle feeding does not provide all necessary nutrients that are gotten through nursing.
I do not see any reason to separate the baby from mama and siblings in the first place, as eventually he / she does need to learn how to eat by itself - Unless mama is not taking proper care of the babies and / or it is something temperature / disability related, I feel like moving baby bunnies will stress them out in this maybe what's causing the discolored feces
What breed(s) are the parents? As you may be aware, in dwarf breeds, there is a thing called peanuts which occurs when 2 true dwarfs are bred and unfortunately they do not live for much longer been a week or two. This is very unlikely seeing how long it has survived already, but knowing some background on the parents may help!
Hopefully you'll be back home and can get the runt back with it mama and sibling ASAP. As well as getting it nursing properly and hopefully growing stronger. As others mentioned, getting in touch with a local vet is a very good idea and I encourage it strongly. The baby has made it this far, and is clearly a fighter!! Seeing as your rabbit did not have a large litter, she should have plenty of milk, but if not, please check out my other forum which includes a list of natural remedies used for breeding does to increase milk flow.
Best of luck!