I have been waiting to post until I had good news, but good news seems to be a long time coming so I thought I'd better give an update now anyway.
My vegan diet has gone by the wayside, for at least a year until this protracted withdrawal is more under control. I tried rice protein powder, the third type of protein powder I've tried now, and I was immediately allergic to it too. There is nothing in it except rice protein, no additives or flavours of any kind, so it is clearly the protein that is the problem.
I have been experimenting with small amounts of meat at lunch, at that seems to work better for me. I've also found that my body is getting used to the meat again, it does not bother me as much anymore. I definitely feel better the next day after eating some meat, so it will stay in my diet for now.
Right now I'm fighting some kind of throat infection, which has made me sick right through the holidays. The protracted withdrawal is worse again too, probably due to the infection. I don't want to take antibiotics though, because I've heard those really aggravate the protracted withdrawal as well. I'll just wait it out and see how it goes.
Happy New Year everyone!