Can caffeine and antihistamines have an affect on Hair Transplant Graft survival and the final outcome?

There is no reason to believe caffeine or antihistamines will affect the results of your surgery. If you are concerned, it is best to contact your doctor.

At 16 days post-op your grafts are fairly well established so things like caffeine shouldn't really impact them. Even in the first few days they shouldn't have substantial impact.

I am not aware of any study that shows caffeine or antihistaminies having any effect on transplanted hair. Of course avoiding medications or habits that result in vasoconstriction is always a good idea post operatively.  You are already more than 16 days out from surgery and the grafts are well seated and have re-established their vascular system.  You may resume all of your pre-hair transplant medications.