You wrote "when talking to somebody within your in-group can probably only be an insult", but it's not.
Being spoken to with humble speech per se means it's no longer the in-group **in its own** relationship (in other words, it can in another relationship). Whether one belongs to the in-group or out is relative and depends on cases. So it still works as a humble speech, though, of course, since it's abrupt and unusual to use humble speech among close relationship, it would sound joking.
So, nobody takes it as an insult as long as they have common sense, though series are a dated expression anyway.
As for paraphrases, I come up with for your son but I don't for the rest.