“Being (even marginally) protected from the elements by them inherently counts as a benefit, even if it’s very hot out.”
The shirt and undershirt each provide better such protection than the 4 cornered garment. Wearing another layer when it is very hot is akin to eating when you are completely “stuffed” (Achilah Gasah).
No fulfillment of Afikomen on Pesach Seder night and no major violation of Yom Kippur