Can a missed shot still execute?
After some investigation, it seems Repeater only works on successful shots but if you have Stock then that damage can trigger Repeater bonus and execute the enemy.
Source: https://gfycat.com/PreciousOffbeatArcticwolf
Reddit thread: https://www.reddit.com/r/Xcom/comments/44oxea/stock_repeater_flawless_execution/
It used to be possible for Stock+Repeater to trigger a "One-Miss Kill".
However the Alien Hunters patch (May 2016) has removed this possibility. So if your XCOM2 is auto-updating, this is no longer possible.