I don't believe I missed any of the concerns you mentioned, I think perhaps I did not make myself clear.  Of course the mortgage is not worth as much in the secondary market, but from the facts we have been given, we don't know by how much.  The loss of priority in the event of foreclosure obviously plays into the equation, but again, we do not know from the facts presented how severe the additional exposure is.
The point I am trying to make is that we are dealing with a $15,000 loan that is not in default.  The maximum loss would be the same $15,000 plus any interest, and loss by the lender by not being able to sell on the secondary market for the amount they anticipated.  It seems that at this time the only realized loss is the profit the lender could have made by selling the loan at an expected price.  Rather than taking this to court, it seems wiser to me for the parties to concider how important their inter-relationships are.  If they value the relationships, they should be able to each kick in a few dollars and/or promises and move on to the next deal.  If one of the parties feels it is not worth preserving a relationship, they can all waste some money having the courts determine the proportionate responsiblity.  I just don't see the point of litigation now, the $15,000 is not gone yet.  It seems better to have everyone do some butt covering for the moment and see how it plays out.