Glass 2 glass bonding is usually done with Ultra Violet setting glue, you can get it from Maplin and many other places, car spares shops usually have it for rear view mirrors. Daylight is sufficient to set it but a UV lamp makes it a lot quicker.
What I would try (and I mean I would try, I don't know if it would work) for filling behind is that stuff they glue plasterboard to walls with these days, Dry Wall Adhesive. The biggest problem will be pushing it in place without breaking the join again and trying to get the mirror flat not bent.
Have you priced up a replacement mirror, it might not be that bad from a glass place, phone around, slight risk of a different tint.
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