OP, it’s great that your child has a wide, varied and balanced diet.
No one deliberately thinks ‘I must fund an unhealthy food to feed my child’. Well, except you, recently
. They choose from the food in offer and buy what they like and afford.
Others, when at other people's houses, go with the flow. If a slice of cake is on offer, they don’t make a fuss, it is just part of the food on offer. And in moderation a piece of cake, a couple of biscuits or a bit of chocolate is fine.
Just don’t make a big fuss about it being unhealthy or healthy. Your DS will not associate it with a relative any more than one relative serves curry, one salads, one beans on toast. It’s all part of the food palette.
You have answered your own question: you are over-thinking.
(And I avoided ‘fruit leather’ as basically dried sugar)