So far NOBODY has been able to provide details of a case where an airline has actually taken ANY pilots to court, with regards to such bonds! (where the airline have won such a case)
Can't post specific details here but I can assure you that Singapore Airlines has won court cases involving bonds, however they also did lose one in the USA which they chose not to take to appeal. I also know of two people who, instead of jumping ship, went to the office and negotiated a settlement that was exceptionally fair and easy.
In the cases you quote the amount of bond, E15,000. and period of return of service, 21 months, seems fair. Bear in mind that after only fifteen months on type you are of limited employability anyway, a different story after thirty six months on type.