I'm so sorry about all of this! I'm no expert at all, but my 5 y/o guinea pig has had persistent bloat for around a year now and I've learned a couple ways to minimize it.
The most important thing as mentioned by everyone is the massages, when guineas have bloat, especially persistent/severe bloat you will be able to feel the bubbles pop and move. I've tried using massage devices in the past and they work well too! I do prefer doing long strokes from around the rib area to the butt to help guide it out and have found that it works, and all of my vets have approved of that technique too.
Another thing that I'm not sure if anyone mentioned, or your vet touched on is veggie intake. If you can, cut out almost all veggies/fruit. They tend to make the bloat worse. Hay + pellets should be okay, I also give Bene-Bac (a probiotic) to my guineas super often. If poops get bad, feeding them their own poops, a probiotic, and cutting out veggies should help. Critical care or a supplement is great if your guinea isn't eating like you mentioned. One of my guineas went through the same thing but be careful because with bloat-prone pigs; syringe feeding can have air bubbles which causes more gas build up in the stomach. Also, if your vet hasn't checked yet for any ulcers in the mouth I recommend getting that checked! It's a bit unlikely in your situation but lots of syringe feeding, or even a piece of hay could cause ulcer's in guineas mouth's which may make them not want to eat as well. (Unlikely at this point in your journey but has happened to me)
As for medicine, I live in the United States so a lot of the names/popular medications are different. I use a combo of cisapride, meloxicam, and simethicone. I've heard some mixed things about simethicone but it has helped in my persistent bloat journey. It was prescribed by a vet but many vets also say you can get at a drug store, just some baby simethicone if you're ever unable to access the vet in time. I've heard gripe water is also great too but I have no personally experience using it!
I wish you all the best and please remember this is not your fault at all! You're doing your best and I'm sending you and your piggies all good energy