Hello, you have answered your own question. "if one can use a mirror to light a fire from dry leaves etc when one is playing at survival"
A normal mirror in a garden does not "play at survival" whereby you hold a magnifying glass or convex mirror at a specific angle for the sole purpose of igniting previously dried material. You need to focus the heat on a specific point to start a fire, your average mirror is flat, you need a convex (or parabolic) mirror to focus the suns rays to a small point. Even then, what you are pointing it at must already be dry and combustible. Since mirrors are mostly flat, that is why you do not hear of fires in gardens caused by mirrors.
Mirrors can be quite good at making a garden look bigger than it actually is, they should be mounted at the opposite end of the garden entrance, so anyone entering the garden sees the reflection of the garden before they realise they are infact looking in a mirror. It has been done for centuries with no problems.