I've not read all of it..... so sorry if this has already been covered....
Have you done food diary/elimination diet/TED? Non-IgE allergies won't show up on anything, yet nil-by-mouth for 48 hours seems to fix you? I'd be tempted to do a total elimination diet at this point, as it could be build up of an allergen setting you off, rather than one off eating? The mucous, I'm sure you're aware is used by your gut to try and protect itself, and very common in allergy poos. You would need to do elimination for a good 6-8 weeks to clear your system, and things could get worse before they get better as your system clears. Every time you have a reaction to an allergen, the reaction can get worse.
Medically, nothing would show up on any tests.
You must sort out your childcare situation. You're saying you're considering emergency Foster care, and that's fine if there's genuinely no alternative, but you have an incredibly debilitating illness. You need a back up plan as you are potentially not safe to look after children this young. I had a really bad migraine/suspected stroke a few years back, and they sent an ambulance basically to look after my 18m and 3½ year old. I said ambulance was OTT and they said no, not with 2 young children. They take this kind of stuff really seriously. Had I needed to go to hospital urgently, one of the paramedics would have stayed to keep the children safe. (My husband was half an hour away at work, on his way home, but they weren't happy about the children.)
I would also expect my husband to understand how serious this was if I was suggesting emergency foster placement. If the roles were reversed, you'd be home ASAP. And emergency childcare is a fully legit reason for leaving work.
Also, the in-laws would help DH? But not you? By helping you, that helping DH and THEIR grandchildren. Would they really not want to help keep the children safe?
Even your friends who have just had babies would likely help in an emergency, I know I would! Admittedly, it would be an 'I'll just keep everyone alive, fed and watered, and if they aren't crying, that's a bonus!' Offer, but most people will help if they know you're desperate.
Best of luck and good luck sorting things out for the future.