Cando
"Perhaps he is useless but could this not be dealt with more positively?"
What is not "positive" about giving the man a chance to step up properly for once, not patronising him? That would be considered a decent chance in a work place. The only reason you would see this as not "positive" is if you think he really shouldn't need to do it, and it's completely legit that he passes responsibility off on someone else all the time.
MN is mostly composed of the "someone else"s and many of us are sick of it.
"2 - Why is working as a unit enabling him?"
Being able to offer basic hospitality in hour own house is an extremely rudimentary standard of being a reasonably domesticated adult. Offering food, drink, seats, chit chat, is something that will happen often in most houses, especially ones with new babies. If you saying that it is fine that whenever activities like this happen, it is ok that the woman has to take responsibility, you are shoving a lot of respsonsibility on the woman and that is ok. "acting as a unit" means "he doesn't have to take responsibility". That really isn't fair with somethings as basic as this.
" Perhaps he genuinely doesn't have a clue about this sort of thing? "(In which case he needs educating)"
That's not the woman's job. The way I learnt to deal with guests was by doing it. I did it to help my mum and then from 18 I did it in my own place on my behalf. I am sure I did things wrong at times, and yes maybe there were some ignominious dashes to the takeaway, but that's life. If he is old enough to have children, he is old enough to get on with this.
"3 - What a wonderfully passive aggressive way of dealing with the situation, can the OP not talk to her husband about it without involving the guests"
Well perhaps she has tried that and is now reinforcing it by putting it out there in public that "in this house it's not just me on domestic duty". Women like this have a lot of habits to break: their own; their husband's; their social circle. They need to send big signals.
"and why would you refer your guests to talk to your husband (sorry I forgot they are his parents so by definition, his problem)"
Why not? Why do all guests have to be looked after by a woman? Why is it even remarkable that any guest, however related, would be hosted by a man?
"4 - tinkly laugh - need I say more?"
Finally - fair enough