We have a pc that is getting very sneaky with his sacred flame application. Its fun outside-the-box play but if he carries a hand mirror or uses scrying spells its going to become far easier.

Can a mirror allow use of sacred flame around a corner?

Can a pc scry to the other side of a door and use this sight to activate sacred flame? 

General Targeting Rules:
> A target with total cover can’t be targeted directly 
by an attack or a spell, although some spells can reach 
such a target by including it in an area of effect. 

Sacred Flame Text:

> Flame-like radiance descends on a creature that you 
can see within range. The target must succeed on a 
Dexterity saving throw or take 1d8 radiant damage. The 
target gains no benefit from cover for this saving throw.

Sacred flame specifically states "that you can see", it is also confusing what the "area of affect" might be.