It's not 'manners' though, it's spinelessness, meekness and people pleasing. Absolute lack of character. That's definitely not a good thing, in case I wasn't clear.
How many times I read on here: my MIL gifts me men's size XXL boxers for the past 15 Xmas, I'm a woman size 10, I just smile, thank and wave. OR: I visited my friend and was offered coffee, I don't like coffee, but she was out of tea and insisted we'd have a cup of coffee, so I accepted it, smile, grin&bear it and was sick afterwards. Etc etc etc. So many examples
That's not 'manners', that's stupidity. Same here, OP thinks the girlfriend is a rude cow. And seethes quietly, complaining here, to her husband, but not saying a word to her or the son. Why? Because you're ' a gracious host and don't want to alienate your son'? No, it's because you don't have the balls. Just own it.
No Italian mother would sit meekly in her own house being attacked by some girl, she'd give it to her both barrels, loudly and clearly, the girl would run bawling. And no good Italian son would sit quietly whilst his girlfriend is thrashing her mother.
I'm Eastern European, only the Dutch rival us in the directness stakes. I'd tell her straight in her face, to pack in the lap-sitting and munching each others faces off, we're not a brothel or a porn set, we have our rooms for that, no one ones to see their raging hormones at the breakfast table.
As for fat Brits and shit food, well that's the honest truth. It's not 'Italians', everyone in Europe think that. And why? Because it's the truth. I couldn't with good concience deny it, so I wouldn't call her out on it. Brits are fat, not good looking (being polite) and the food is shit. Jar sauce, countless cokes and instant coffee. And for an Italian, no less? That's just abysmal. But shit, I'm as blunt and ballsy as they come, but I wouldn't cook pasta for an Italian, that's something, hats off to you for that, OP.
And again, she's factually completely correct in all her assertions, but if you don't like it, just give it to her straight: we don't comment about people's appearances in this house, it's rude to critique host's country in this way over here, my health, food and weight is not your businesss, so STFU.
God help you OP, if the next girlfriend will be Dutch of EE. Also tall, slim and beautiful, but far more opinionated and not afraid to say it. At all. You'll kill yourself if you keep quiet all the time, this way.