I didn't have much formal grammar taught at school - we did nouns are naming words, verbs are doing words, adjectives are describing words, but that was about it. I learnt foreign languages to A-level and learned a lot more through that. But I also came from a family where we looked things up and discussed the etymology of words, and my grandmother wrote me letters saying things like, "practise is the verb, practice is the noun." Plus we had no TV, so I read all the time. But most people don't come from a background of language pedants...
I don't reckon many boxes will say "baubles" - I think most will say things like "silver tree decorations" or "multi-coloured glass balls". I think most are in clear plastic boxes. I'm not going through the box of decs to check, mind you. The only one I can remember is the one which says 1/6 and was bought by my parents when they were first married. The only writing is the price - most of it is a clear cellophane window, yellowed with age.
I don't mind things like Chester draws too much, because I know what it means. I do care about things where I can't work out what it is meant to be, and I do care when it's in a more formal context such as a published book, an advertisement, a CV or something where it would not be unreasonable to have someone else review it before it's sent out to the wider world.
I don't think Plato's cave would be general knowledge to many - I don't think many of my colleagues have even a basic knowledge of classical philosophy.