Good Morning, 
I just realized that this threaad had been updated. Sorry for responding so late! 
So.....here's the thing. It seems to me that this could be a couple of different things, and it would be really good to get a better idea of exactly what the problem is before throwing a bunch of medicines or treatments at it. You wouldn't happen to be able to take a fecal sample to a vet for analysis, would you? Many vets that won't treat chickens will still do a fecal check when you tell them that they will be looking for the same things as they do in dogs. Ask them to check for worms and Coccidia. I recently had a sick hen who had dirrehea and would not eat. She had lost a lot of weight, and I was really worried that I was going to loose her. I was pretty convienced that it was worms, but I had my vet do a fecal check ($15) and it was Cocci. Some level of Coccidia is normal, but when there is an overpopulation it can kill them. After treating her with Corid for 5 days she is 100% better. I wasn't expecting Cocci because she is 11 months old, but they can always get an unhealthy balance. 
I know that in Storeys guide they advise against worming to build up resistance, however, a lot of folks here still choose to worm twice a year to be on the safe side. The thing is though, if the problem is not worms....wormers can be very hard on a bird that already has a compromised immune system.  I would want a confirmation of worms before I decided to worm a sick bird....and, if it is not worms and you are able to get her well, once she is recovered you can worm your whole flock. I used Wazine in their water, and then followed up in 2 weeks with Valbazen (You can get both from 
tractor supply, southern states, many feed stores, or order online). 
The "throwing up" really bothers me. It makes me think crop issues. Even if her crop is not fully impacted, it may be getting slow or sour. Again, chickens don't throw up, so liquid coming out of her mouth means that things must be backing up in her crop. The contents will continue to rot or ferment and poison her. IF this is the problem. There are a couple antifungual medicines that some people use, they have been used in women to treat yeast infections and for chicldren with thrush. I think the two are Nystatin and Diflucan. Basically, you want to continue feeding only very easily dissolved foods. Mashed, hard boiled egg yolk (no whites, not scrambled because they aren;t as easily broken down). Yogurt, her crumbled ground fine in a food processor. The general rule is anything that will dissolve and settle to the bottom in a glass of water in five minutes is all you want to give them. You take the food up at night so you can see if her crop is emptying, which since she is pooping a little, it seems like at leaast something is getting through. The other thing you want to do is try to massage that crop to break up the contents so they can pass. Massage serveral times through the day if you can. This is just what i remember, let me dig up my links on slow/sour crop and post them here.......even though the fluid isn't stinky yet, it may just be that you caught it very early, which is good! The the other thing to note here is that a slow crop can be a symptom of another problem all together. 
The other possibility would be respiratory issues, although this doesn't seem as likely. If it were respiratory, I would have a hard time pin pointing which respiratory problem it could be, unless there are more symptoms. There are a ton of respiratory meds out there, but certain ones are more effective for one type of sickness over another.
I would be very tempted to call Peter Brown of first state vet supply and have a phone consultation. He asks for a donation, or for you to purchase something from his website, which has tons of chicken meds/supplies. He is very helpful, and stayed on the phone with me for an hour explaining how to tube feed a chicken. I highly recommend him. Here is his website 
www.firststatevetsupply.com He sells both Piperazine 17% (same thing as Wazine) and Valbazen, so you could always just order your wormers through him, and get the consultation for free. 
Ok, I am going to dig up all my slow crop research, and will update with links on here for you to read. In the meantime,  Can you give us as many details as possible as far as Butters and her flock mates go? Have you ever had any illnesses, what kind of food do they eat, do they free range, do they always have access to free choice granite grit, have you had any problems with sneezing, nasal drainage, rattling chests, etc...check Butters over really good for us, and see if you can find ANY otheer symptoms of respiratory. Any bubbles in her eyes, any nasal drainage, etc....? I'm leaning towards crop issues on this one, but again, it could be several things. 
How is she feeling this morning? Did her crop empty last night or is it still squishy? Is she loosing weight yet, or does she still feel healthy? No more "seizures", right? I'm hoping you caught this early enough that we will be able to get her better, and she'll be strong enough to fight. I'll edit to post crop links below. 
Andrea
Okay, here's what I have so far: (From my library of Threehorses old posts)
https://www.backyardchickens.com/forum/viewtopic.php?id=248715&p=1 This thread has some pretty good info about what to feed and how to do crop massage with a little Organic Apple Cider Vinegar to help break it up and stop fungal growth. Unfortunatly, the hen died....and the OP did not update much with what she had tried, but there's lots of good info. 
https://www.backyardchickens.com/forum/viewtopic.php?id=207340&p=1 Information on Nystain and Diflucan along with good dietary info for crop issues. 
https://www.backyardchickens.com/forum/viewtopic.php?id=262329 Information on crop issues being a secondary effect of botulism and or/worm infestation, discusses gently flushing along with crop massage...