Plutos, to answer your questions:
1. You don't lose any allowances neither in case of a mortgage nor in case of rent. NONE OF THEM.
2. Legal aspects: you have to understand that foundation of your possible residence in Qatar is based upon the employment in QR. QR bears responsibility legally and financially for you for the duration of the employment. Once the employment is seized you're on your own. In order to leave Qatar in good terms you will have to be cleared by the company as well as any creditors. Obviously having unpaid debts with local bank (the mortgage) the chances to be released do narrow unless the property will be sold for a price covering the bank's expenses. Once clear it will be advertised in local newspapers MR. X is leaving the country at such and such anybody having any kind of legal claims do report to.... blah blah blah. Once that is done you can go.
Reality is that people do take loans and just jump ship...
Just to give you an idea about how it looks. Good luck.