If my opponent used Magnemite's "Mirror Shot" attack last turn, can I use the effect of Will to choose heads for the effect of the attack?
Yes, that is considered an effect of an attack, so that works.
Source: Cosmic Eclipse FAQ (2019-10-18), TPCi Rules Team (2019-10-31)

Relevant cards

Note: All applicable cards may not be listed below.
SM12_208
SM12_208