Just a few considerations to think about when you are sticking things like mirrors to walls:
Are you sticking something to the wall or to the paint or other surface on the wall?
Is this paint or other surface likely to detach itself from the wall after you have stuck something to it?
eg, I'd like you to think about sticking something to the paint that is already peeling off the wall. Not a good idea, is it?
Similarly with wallpaper that could get damp and begin to peel off the wall. It's not a good idea to stick something to a wall covered in that, is it?
And how about covering the entire rear surface of a mirror with an adhesive film of wide cello-tape, so that if it ever gets broken all the bits of mirror stay stuck to the cello-tape? (This idea is used for those massive sliding door mirror wardrobes , such as they sell in B&Q and Homebase)
Could you usefully soundly fix anything (eg, an aluminium, chromed, brass effect or plastic bar or strip) to the wall underneath the heavy object (ie, mirror) that you want to stick to the wall to help support its weight and thus make the whole thing a safer fix?
If the mirror is large could you soundly fix something to the wall above it so that it has absolutely no chance of falling forwards and then dropping off if it comes unstuck?
Have you seen these fairly cheap
Fischer Mirror Fixing Kit fittings?