Yeah I would definitely be careful with the B vitamins. I can't tolerate any of them at all. As you know I have issues with MCS and b5 never did anything for me but worsen brain fog and make me more sensitive.
Like Mikey said, best thing to do is start very slow, one by one and monitor reactions. BABY doses though. Like 1/4 of a tablet! Because the more severe reactions you have the longer it will take your body to calm down, and you want to be able to actually eat before anything else :)
Vitamin C will help as it is also an antihistamine, but try and get a good brand that isn't derived from Genetically-Modified-Organisms corn. I like the brands Perque and Nature's way Alive. I switch back and forth between them, some days my body likes one, other days the other.
I've healed quite a bit from actually NOT taking supplements believe it or not. I didn't take anything for months as my body was way too sensitive for anything and I was more concerned about not starving to death. Just in the past two months I have started back on vitamin C and magnesium oil at night. I take quercetin, but not every day. I prefer to get my nutrients from food. Golden delicious apples are excellent sources of quercetin (I eat 2 a day average) and white potatoes are one of the best sources of vitamin C there is (I go through 10-15 lbs of them a week). Both are failsafe foods.
So I can attest and am a walking testimony to the fact that you do not need supplements to heal, just good quality food.
Interesting you have HHV6. I do too. I also took a round of Antibiotics and 3 months of valtrex before I got severely ill. Been trying to find some research that will possibly connect the virus to AF/MCS etc. No doubt it is a factor because viruses are a chronic stress on the body.