To wolf, As to when the obligatin to pay;
It used to be minhag at the engagement- ewhen they had a real vort with tnoim.
In todays time the obligation is when they actually marry.
To Gavra, actually there is a special haloche that making a shidduch obligates you even if no kinyan is made.
There are similar rules regarding “sirsur’ being a broker for anything.
There is a siman in chosen mishpat dealing with this. (Im embarrased to say i cant remember which.)
To of course, if the parents dont pay the couple is obligated.
Speak to the rov, and then send a note to both michetanim (c.c. to each other) then send a polite note to the couple, wishing them siyata dishmaya. (Consider; Ask them if they would like to consider it as tzdoka)
Actually you probaly just want the decency of recognition.
In which case say you feel hurt they didnt invite you, and perhaps they can send you a note and a token of their appreciation, and an invite to the bris/kiddush