As this is about your health, you shouldnt worry about the person who can give you the information and what they will think about you.
I think with all the stupid requests people can make on a trip, this is not one of them and I would not consider you high maintenance. High maintenance to me is making 274 requests about products you need to have, like that you can only eat dairy from a goat farm in Italy where the goats are milked by the light of a full moon
As producers can change the ingredients of their products, I wouldn't even rely on
DCL to ask the question on forehand, but go to the spa and ask them, then you are sure. I would book the appointment now with a remark about your allergy.