Translation:My friend, whose dad is a lawyer, is in jail.
They're optional, but I'd say that in general you'll tend to use commas more often if the information in the subclause is less relevant to the sentence. You'll also tend to not use commas if they disrupt the flow of the sentence, as Mannen, vars fru är polis, är här nu would do.
I know it's just a typo, but it's sitta inne, otherwise it sounds weird because it's the wrong preposition.
And you are absolutely right: we can't really accept all versions that kinda mean the same thing - that'd mean absurd amounts of extra work for us. It's just not feasible.
Accepting alternate terms for family members is unfortunately a logistical nightmare. For an explanation, please see the top question of our FAQ:
In short: you're not wrong, but it's not realistic to add everywhere.