Yes similar issues! 5 y o who is rude; a lot of this seems down to cultural difference (I found it helpful when finding out literal translations - it’s all ‘I want food’ - no nuance, please/Ty not used as commonly as here. So when he says I want drink, this is a normal way of asking, just translated. So I’m tolerant of language. And now just find rudeness in other things!!!) but he is incredibly demanding and takes my children’s toys to his room (we have provided loads of toys for him - their room is big enough to play in). I insist he asks for things politely, and ask him to eg. clear his plate - I am the only one who does this.
Food - lots of sweets, chocolate with most meals. He is tiny. The meals mum feeds him are v healthy and filled with veg so I guess it all works out; it’s not really my business.
Bedtime - I know you say you have resolved the issue, but are you happy with this arrangement? It turfs you out of the living room that you are paying for. I have a no screens after dinner rule for my children which I initially I didn’t extend to them (diff family, diff rules) but now do as I found we were being turfed out. This has proved genius as there is no particular desire to go in the living room for him after dinner! We do have a playroom which is also filled with toys.
I am very clear on bedtime; I tell him it’s bedtime and he needs to go up. Mum goes up with him and does stuff on her computer when they go up. I have put a tv in their room and pay for a skybox and I am aware that he watches tv up there when I send him up. We have an older Ukrainian guest child who puts herself to bed; she is no bother and stays up with our children, but again goes up when I announce bedtime for all. I don’t agree with their bedtimes being so late, but again, it’s none of my business and is a cultural difference.
In your position I would just start to reclaim the living room. Explain that you want to watch something unsuitable for children which will force the mum’s hand. Appreciate that her child doesn’t want to go up alone, but this should not be your issue; mum should go up too.
Hope it gets easier for you.