Fixing a poor measurement of drywall toggle anchors
The structure of the wall is gone in the direct vicinity, so moving the anchors you have will not work, and spackle is more a replacement for the paper on the drywall, than the gypsum itself.

Most useful, I think:


Plan A: How to wire a picture, using 2 hooks. The hooks do not have to be level


https://www.flickr.com/photos/some_guy/16751622487/

Other Options:



mount a small piece of sturdy lumber, oak 1x2 molding (predrilled) would be fine  as parallel to the floor using the studs.



Drill for the screws to catch the d-ring on the mirror.





A low profile hook/bracket that you can screw into the anchor and 'swing' far enough to catch the d-ring on the mirror. A 45 degree angle on the hook will double the holding force on the anchor, so no more than that. 



A loop of piece of picture hanging wire, quick link, modified simpson strong tie or the like is also valid here.




3.Get the anchors you have out of the wall


Cut (drywall saw, hacksaw blade, steak knife, ginsu or similar) the 1/8 over to where you need.

Get a couple of 'real' spring loaded toggle bolts, with the longest arms possible. You can get shorter screws if needed.

Build a washer 'sandwich' with the largest fender washer possible to go against the wall, as many normal size washers as needed to hook the d-ring, and then a larger washer against the toggle screw that will keep the d-ring from bumping/sliding off the front of the bolt.

With the exception of the washer closest to the screw head, they do not need to have the 'right' size hole in them, just enough to not fall through each other.

Expand, by cutting the hole in the wall just large to fit the toggle through.

Tighten as needed, and then get the mirror on the wall to prevent anyone from seeing the mess (I have this final activity down to a fine art.)