Is it customary for nobles not to eat as a family or is it just in Manga that this occurs?
If it isn't customary then where is her family at? I mean throughout the story so far we have seen her interacting with the staff but not her family! If the slime was able to eat with her family even if they were afraid of her or had the same nasty personality as Jelly then the slime would have been able to connect memories to actions (i.e. proper eating manners), because the staff is too terrified of her to correct her manners.
That's a fair and interesting question. Noble practices greatly differed depending on the era and region. But for the most part, it should be with family, if not an outright communal affair. Masters/Mistresses of the household could usually have the option of eating alone and in private though...I mean, it's their household. So assuming the writer thought that far, it could hint at her host's status.
Otherwise it's quite impractical for meals to be served to one member of the family at a time. In many nations, while people sort of had multiple meals a day, there was really only one substantial meal, aka dinner (which wasn't always in the evening/night). And it was usually a huge one, meant to last them for the whole day and could take hours. Imagine catering that to multiple personages separately.