My question surrounds single guests. If we choose to allow a single guest to bring a guest, understandably, we would address the wedding invitation to indicate this. But to my mind, a Save the Date is meant to simply reserve a date for the people WE are specifically inviting and it should therefore be addressed to our specific guest list... therefore, my question is, does a Save the Date (not the wedding invitation) to a single guest need to say "Jane Doe & Guest" or just "Jane Doe"? What is correct, given that Save the Dates are fairly modern?
You are correct that these are fairly modern, so there isn't as many 'rules' for these. You could include the 'and guest' if you wish, but it isn't mandatory. It is entirely your option.