But there would have been no other reason to have a bell in the nursery that rings in the servants' hall, other than for the nanny to summon a (lower) servant, to bring up more fuel for the fire or clear the things after the children's meals etc. One of the housemaids or the 'between maid' (whose job was waiting on the senior servants like the butler, cook, housekeeper, and waiting on table in the servants' hall) would have answered it.
Mrs Hughes appearing instead makes no sense (apart from the writers clearly wanting a trusted figure, not one of the junior maids, to show up and hear Cora's wrath!) Apart from anything else, it would have meant her climbing up several flights of stairs to the nursery floor, and she's no spring chicken, and she appears in feasibly fast!
Merguez I think that's 100% true, unfortunately. Goes along with the new lines of merchandise, featuring inscriptions like 'Kiss me, before I get very cross' (Lady Mary)...
I have to keep reminding myself Fellowes also wrote Gosford Park!