You’re way over thinking this, IMO.
Why not just treat each address differently?
Oh…wait. I see now.
You want one invite but different RSVPs? I don’t know of anything off hand that’s going to handle that unless you do the RSVPs and the invites separately.
I took my numbers spreadsheet and broke it into 3 tables, based on the number of inviteesm (1, 2, or 3) at each household. So for example, I created a table called "three" and each record had the address of the household and the three names of the invitees at that address. Then I created different response cards in Pages (I'm too new to InDesign), one for single invitee addresses, one for double invitee addresses, and one for triple invitee addresses and I will perform a data merge on each with the appropriate tables.
InDesign's Data Merge literally plops data onto a page wherever the formfields are. It isn't intelligent i.e. doesn't have "triggers", such as "if address = xxx then... else...". Without the aid of third party plug-ins, the Data Merge function is pretty simple and doesn't perform database query results such as the the example given by the OP.