i used freeze egg whites a lot when back in my country since a lot of our dessert used a lot of yellow egg , but never put in ice cube , usually i use small plactic bag that been weight on some recipes that use egg whites.
if you freeze it they can store for a long time , but i never used it if over month.
this one i'm not 100% sure about if frozen can still make meringue but i think they can. because some of my recipes that use egg whites that i had frozen , they all needed to whipped into peaks so i think they can.
i hope that help