My ds also has issues with eczema and heat and yes, I think the chemicals in the car seat cover are an issue too. Inititally we had problems with the straps, so I bought terry cloth covers and that helped tremendously. If you haven't done so already, I would try washing the cover several times in a fragrance / dye free detergent - Purex works for us. I would avoid using Dreft because it can be highly allergenic. Or if removing the cover seems too daunting, consider buying some cotton receiving blankets (flannel would probably work great too) and cover the seat with them (cut them up so straps can fit through safely). That can also help with sweat absorbtion where the synthetic material of the car seat cover may not allow for that.
We also found that increasing my son's liquid intake before he was going to be in a hot situation would force him to sweat when he got too warm. Otherwise it was as if he didn't have enough to actually sweat out so the toxins were trapped in his skin - does this make any sense?
I would also try changing your sunscreen in case that is what is causing the issue.
You may want to try taking one step at a time to see which factor it is. It really helps in the long run. I understand wanting to be proactive rather than reactive. Our doctor was not of that mindset just loved to hand us prescriptions for Elidel
But that is why he is not our ped anymore...
PS Food can always be an issue, but it sounds more like a heat issue to me if since it occured on several occassions and the food was probably a one time issue?