A 86 partial:

According to pg. 198 of the PHB, "When you have temporary hit points and take damage, the temporary hit points are lost first, and any leftover damage carries over to your normal hit points.". No option is given: The temp HP are lost first.

Similarly, from page 115, "Whenever you take damage, the ward takes the damage instead.". Again, no option is given.

I don't know the answer for shields, but I think that there might be a rule somewhere that you can simply choose to be hit by an attack, regardless of AC.