I guess I should comment on this one
I have, over the years, retuned fish that have taken a bait deep, with the hook in situ. Some of these fish have been large and recognisable, and I have caught them several times again over the years. (Pike and Carp). I have also taken Pike that have had several traces down there throats
with 2 large trebles attached. These fish continued to feed, even with all this ironmongry in there gobs. I have then removed these traces and sometimes the hooks have rusted away, other times they have not, but the point is, the fish has survived and continued to grow. If it had been killed, then it would have no chance.
My view is this, if the fish is bleeding from the gills (talking trout now) I would knock it on the head, if the fish was not, then I would snip the nylon as close to the fly as i could and release it. I do not think a fly on a size 12 hook is much of a problem for a trout as they continuely take sticks, stones etc when they feed and can then "cough them up" at will. Open a rainbow from a water that contains sticklebacks and you will see even the spines from one of these do not pose a problem.