Fire resisting doors are different than fire escape doors. An external door (some exceptions) in the main would be fire escape doors and therefore not fire resisting in themselves.
Therefore your second paragraph is kind of answered also.
Yes softwood can be used. The door arhieves the raiting, ie 20, 30, 60 minute etc, but is useless without a fire rated frame. 25mm stops with an automatic door closer achieve a suitable fire rating, but it depends on the rating you are after.
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