From what you're wanting to do (dual purposes) I think the best solution would be a laser sight for CQB/self-defense and a 1-4x or 1.5-6x for the distance shooting. The laser would mount on the fore-end and have a press-button to activate (think Crimson Trace grips) so you could have it and the scope mounted at the same time.

That said, I don't know anyone in their right mind who would choose or recommend an AR-15 as a suitable home or self-defense weapon. A shotgun with a standard stock and 18.5" barrel is unwieldy in the confines of a home for well trained and experienced operators and a 16" AR15 with a collapsible stock isn't that much shorter, so to think it would be a good choice for a novice or less experienced shooter for home/self-defense purposes is a little ludicrous. Set up the AR for how you want to use it and get the wife a suitable handgun for self-defense.