"You have to respect his decision to become a priest."
Translation:Du måste respektera hans beslut att bli präst.
"Ni måste respektera hans beslut att bli präst" is an accepted translation, but should it be? The English sentence mentions "a priest", not "priests".
More generally, do Swedes always use "ni" when addressing several people (e.g., as an author or broadcaster) or do they sometimes use "du" as if they are addressing each person individually?
It's possible to use ni as a so called polite pronoun when talking to one person, so that works too.
In Sweden, it can be a bit controversial to use ni as a "polite" pronoun, some people strongly dislike it and some may even be offended. Most people just don't use it that way. In Finland, it doesn't seem to be controversial, but it's never necessary there either.
I think of "för att" as meaning "in order to". If that is correct, then the English for what you are proposing would be "You have to respect his decision in order to become a priest", which wouldn't be right here.
Contrast the above with "You have to take holy orders to become a priest". For this sentece I would use "för att" in the Swedish translation.