We know, we just put the words in the same order as in Swedish to make it easier on learners when translating the other way. It's not incorrect the way it is. But it's good that you're pointing it out, especially for the sake of the people who use the course to improve their English.
That's correct. In a subclause, a satsadverbial, which this is, must go before the verb. Other adverbials go after, like adverbials of degree, time etc.
sentence adverb (or sentence adverbial), which modifies a sentence, or a clause within a sentence, to convey the mood, attitude or sentiments of the speaker,
If I just had known and If I had just known both sound okay, but for me they usually are connected to another phrase. "If I just had known that, I wouldn't have picked up the spoon." Using just also gives the phrase a more petulant/exasperated/defensive tone, as well as making the phrase sound more conversational than literary.
There are subtle distinctions between all the varieties of this phrase, especially when spoken. It's just one of those phrases. :)