I'm still at CAA so will ask the question about the allergies. I wouldn't imagine that it'll be an enormous problem, but as we all know there are some allergies that are more, um, dangerous than others. I'll get back to you with a response later today.
In relation to failing the Class One, I'm not entirely sure that you'd need to come back to try again. The kind of things that they've tested me on today are the sort of things that if broken now, will never be fixed. If I failed today I'd probably resign myself to the fact that I couldn't get the Class One at all.
BUT, if you chose to re-do the exam, I don't imagine there to be any time limit on how frequently or infrequently you can go for the exams.