You can't break a law if it isn't a law. So in your hate speech example it would be disregarded as there wasn't a law to break.
Historic offences are obviously quite well documented at the moment, so there's no statute of limitations as per America, if there was a law against an act at the time of committing it, you can be sentenced for it at any later date.
In regards to decriminalisation, I believe you retain the criminal record as there was a law to break, but they have issued pardons at a later date, as you say, as per homosexuality crimes. I believe they can reduce sentences, but not increase them, for example with marijuana going from class C to class B, they couldn't give you more time, because it has become "more controlled"