I didn't see them curtsying but I did see a Japanese princess curtsying to them for some unknown reason.
Well, all european royal ladies curtsied if you watch the coverage. But it is very common for japanese (and Thai) royal women to curtsy meeting foreign Royalty, too.
I don't understand your argument. What does the size of Westminster Abbey have to do with attending a wedding in Jordan. And why would the in-laws of the British heir to the throne be invited to the wedding reception of the heir of a foreign royal family? I didn't see any other in-laws of either monarchs or heirs; for example, Mary's in-laws weren't there, neither were Daniel's or Máxima's...
So, I guess it has more to do with their 'Jordanian' history (they lived in Jordan for several years in the eighties) of course in combination with their daughter now being married to the future king of the UK, which somehow led to some kind of link to the Jordanian royal family.