Fasting can affect the results in a couple of ways.If the horse is fasted all night, then the blood is drawn, the insulin will be lower, and you may get a false negative on an IR horse.  If the horse was fasted, then fed breakfast, the insulin will increase from the first meal of the day; usually peaks around 2 hours, but can be elevated from the meal (even if it is hay only) for 3 or 4 hours. Therefore, if the horse can't have hay in front of it all night, best to schedule the vet for more than 4 hours after that first meal of the day. 

Your schedule for testing Samson sounds perfect.

Regarding the seasonal allergies/wheezing, there are a couple of options:  

Spirulina at 40 grams per day, plus chondroitin at 6 grams per day; plus jiaogulan.  One can also use Uckele's Lung EQ: