sure, but a lot of this depends on the focus the author intends to have on that investigation storytelling-wise vs how important it is to just make the disappearance believable and consistent with the atmosphere and the general narrative. most i can give out are ideas as starting points, they'd know best what works for them
for instance, in the case you specifically bring up, i can't know what the involvement of characters B or C is with the family, how well of a position they're in to deceive the investigators, what the fake destination would be, how long they'd be sending the letters for, if they'd be willing to disappear themselves, etc., etc.