I think it is fair for parents to ask others to keep the basic framework of what the child has normally (my DS is18m), in order to keep him feeling secure.
We went on holiday to see the ILs recently, in former Yugoslavia, and although we ensured that bedtimes and mealtimes were adhered to (people thought we were amazingly fascist for having DS in bed at 7, but I'm sure they were a bit envious, as theirs went at all hours), we did not try to cut him off from new experiences; in fact, he ADORED being on the farm, eating grapes off the vine, riding on the tractor, etc.
Food, however, was a bit of an issue, and one on which it is well worth standing your ground, for the child's sake. People were trying to offer him fruit juice and things with loads of salt, just endlessly. And what do you know? DS is thirsty for water after he has juice, and didn't like salty things... like prosciutto (bloody hell...)! Poor MIL was trying v hard, but when I decided to make 2 meals - 1 with a little chili and the other a mild Thai red curry, DS ATE them, with great enjoyment! She seemed shocked and a bit upset, and I did feel sorry for her, although I also felt sorry that DS had not always enjoyed what he had been offered previously. ALSO (pls forgive caps), HE HAD A HORRIBLE ALLERGIC REACTION to a crab cake, which I let slip because I was worn out with being so vigilant, and had been beginning to suspect that I was a neurotic control-freak mother. I felt terrible when the poor LO came out in a rash and fever, and had to have 2 antihistamine injections in his bum!
I guess the moral is to keep a look out and don't be afraid to challenge things which are not in your LO's interests (food and sleep are, IMO, the key issues, but your points might be different).
Please don't be afraid to stand up for what you think are your child's best interests.